diff --git a/CHANGES.md b/CHANGES.md
index b02ee1975..2208bd7db 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,9 @@
 
 ### Unreleased
 
+* Add bnew `allow_duplicate_key` parsing options. By default a warning is now emitted when a duplicated key is encountered.
+  In `json 3.0` an error will be raised.
+
 ### 2025-05-23 (2.12.2)
 
 * Fix compiler optimization level.
diff --git a/ext/json/ext/parser/parser.c b/ext/json/ext/parser/parser.c
index c5f300183..627971eb5 100644
--- a/ext/json/ext/parser/parser.c
+++ b/ext/json/ext/parser/parser.c
@@ -35,7 +35,7 @@ static ID i_chr, i_aset, i_aref,
           i_leftshift, i_new, i_try_convert, i_uminus, i_encode;
 
 static VALUE sym_max_nesting, sym_allow_nan, sym_allow_trailing_comma, sym_symbolize_names, sym_freeze,
-             sym_decimal_class, sym_on_load;
+             sym_decimal_class, sym_on_load, sym_allow_duplicate_key;
 
 static int binary_encindex;
 static int utf8_encindex;
@@ -363,10 +363,17 @@ static int convert_UTF32_to_UTF8(char *buf, uint32_t ch)
     return len;
 }
 
+enum duplicate_key_action {
+    JSON_DEPRECATED = 0,
+    JSON_IGNORE,
+    JSON_RAISE,
+};
+
 typedef struct JSON_ParserStruct {
     VALUE on_load_proc;
     VALUE decimal_class;
     ID decimal_method_id;
+    enum duplicate_key_action on_duplicate_key;
     int max_nesting;
     bool allow_nan;
     bool allow_trailing_comma;
@@ -386,15 +393,8 @@ typedef struct JSON_ParserStateStruct {
     int current_nesting;
 } JSON_ParserState;
 
-
-#define PARSE_ERROR_FRAGMENT_LEN 32
-#ifdef RBIMPL_ATTR_NORETURN
-RBIMPL_ATTR_NORETURN()
-#endif
-static void raise_parse_error(const char *format, JSON_ParserState *state)
+static void cursor_position(JSON_ParserState *state, long *line_out, long *column_out)
 {
-    unsigned char buffer[PARSE_ERROR_FRAGMENT_LEN + 3];
-
     const char *cursor = state->cursor;
     long column = 0;
     long line = 1;
@@ -411,6 +411,27 @@ static void raise_parse_error(const char *format, JSON_ParserState *state)
             line++;
         }
     }
+    *line_out = line;
+    *column_out = column;
+}
+
+static void emit_parse_warning(const char *message, JSON_ParserState *state)
+{
+    long line, column;
+    cursor_position(state, &line, &column);
+
+    rb_warn("%s at line %ld column %ld", message, line, column);
+}
+
+#define PARSE_ERROR_FRAGMENT_LEN 32
+#ifdef RBIMPL_ATTR_NORETURN
+RBIMPL_ATTR_NORETURN()
+#endif
+static void raise_parse_error(const char *format, JSON_ParserState *state)
+{
+    unsigned char buffer[PARSE_ERROR_FRAGMENT_LEN + 3];
+    long line, column;
+    cursor_position(state, &line, &column);
 
     const char *ptr = "EOF";
     if (state->cursor && state->cursor < state->end) {
@@ -807,11 +828,25 @@ static inline VALUE json_decode_array(JSON_ParserState *state, JSON_ParserConfig
     return array;
 }
 
-static inline VALUE json_decode_object(JSON_ParserState *state, JSON_ParserConfig *config, long count)
+static inline VALUE json_decode_object(JSON_ParserState *state, JSON_ParserConfig *config, size_t count)
 {
-    VALUE object = rb_hash_new_capa(count);
+    size_t entries_count = count / 2;
+    VALUE object = rb_hash_new_capa(entries_count);
     rb_hash_bulk_insert(count, rvalue_stack_peek(state->stack, count), object);
 
+    if (RB_UNLIKELY(RHASH_SIZE(object) < entries_count)) {
+        switch (config->on_duplicate_key) {
+            case JSON_IGNORE:
+                break;
+            case JSON_DEPRECATED:
+                emit_parse_warning("detected duplicate keys in JSON object. This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`", state);
+                break;
+            case JSON_RAISE:
+                raise_parse_error("duplicate key", state);
+                break;
+        }
+    }
+
     rvalue_stack_pop(state->stack, count);
 
     if (config->freeze) {
@@ -1060,6 +1095,8 @@ static VALUE json_parse_any(JSON_ParserState *state, JSON_ParserConfig *config)
             break;
         }
         case '{': {
+            const char *object_start_cursor = state->cursor;
+
             state->cursor++;
             json_eat_whitespace(state);
             long stack_head = state->stack->head;
@@ -1094,8 +1131,15 @@ static VALUE json_parse_any(JSON_ParserState *state, JSON_ParserConfig *config)
                     if (*state->cursor == '}') {
                         state->cursor++;
                         state->current_nesting--;
-                        long count = state->stack->head - stack_head;
-                        return json_push_value(state, config, json_decode_object(state, config, count));
+                        size_t count = state->stack->head - stack_head;
+
+                        // Temporary rewind cursor in case an error is raised
+                        const char *final_cursor = state->cursor;
+                        state->cursor = object_start_cursor;
+                        VALUE object = json_decode_object(state, config, count);
+                        state->cursor = final_cursor;
+
+                        return json_push_value(state, config, object);
                     }
 
                     if (*state->cursor == ',') {
@@ -1184,6 +1228,7 @@ static int parser_config_init_i(VALUE key, VALUE val, VALUE data)
     else if (key == sym_symbolize_names)      { config->symbolize_names = RTEST(val); }
     else if (key == sym_freeze)               { config->freeze = RTEST(val); }
     else if (key == sym_on_load)              { config->on_load_proc = RTEST(val) ? val : Qfalse; }
+    else if (key == sym_allow_duplicate_key) { config->on_duplicate_key = RTEST(val) ? JSON_IGNORE : JSON_RAISE; }
     else if (key == sym_decimal_class)        {
         if (RTEST(val)) {
             if (rb_respond_to(val, i_try_convert)) {
@@ -1400,6 +1445,7 @@ void Init_parser(void)
     sym_freeze = ID2SYM(rb_intern("freeze"));
     sym_on_load = ID2SYM(rb_intern("on_load"));
     sym_decimal_class = ID2SYM(rb_intern("decimal_class"));
+    sym_allow_duplicate_key = ID2SYM(rb_intern("allow_duplicate_key"));
 
     i_chr = rb_intern("chr");
     i_aset = rb_intern("[]=");
diff --git a/java/src/json/ext/OptionsReader.java b/java/src/json/ext/OptionsReader.java
index c372eb1a8..829e36b31 100644
--- a/java/src/json/ext/OptionsReader.java
+++ b/java/src/json/ext/OptionsReader.java
@@ -53,6 +53,13 @@ IRubyObject get(String key) {
         return opts == null ? null : opts.fastARef(runtime.newSymbol(key));
     }
 
+    boolean hasKey(String key) {
+        if (opts == null) {
+            return false;
+        }
+        return opts.hasKey(runtime.newSymbol(key));
+    }
+
     boolean getBool(String key, boolean defaultValue) {
         IRubyObject value = get(key);
         return value == null ? defaultValue : value.isTrue();
diff --git a/java/src/json/ext/ParserConfig.java b/java/src/json/ext/ParserConfig.java
index 692b3dfaa..ccfc558e5 100644
--- a/java/src/json/ext/ParserConfig.java
+++ b/java/src/json/ext/ParserConfig.java
@@ -54,6 +54,8 @@ public class ParserConfig extends RubyObject {
     private int maxNesting;
     private boolean allowNaN;
     private boolean allowTrailingComma;
+    private boolean allowDuplicateKey;
+    private boolean deprecateDuplicateKey;
     private boolean symbolizeNames;
     private boolean freeze;
     private RubyProc onLoadProc;
@@ -177,6 +179,14 @@ public IRubyObject initialize(ThreadContext context, IRubyObject options) {
         this.allowNaN        = opts.getBool("allow_nan", false);
         this.allowTrailingComma = opts.getBool("allow_trailing_comma", false);
         this.symbolizeNames  = opts.getBool("symbolize_names", false);
+        if (opts.hasKey("allow_duplicate_key")) {
+            this.allowDuplicateKey = opts.getBool("allow_duplicate_key", false);
+            this.deprecateDuplicateKey = false;
+        } else {
+            this.allowDuplicateKey = false;
+            this.deprecateDuplicateKey = true;
+        }
+
         this.freeze          = opts.getBool("freeze", false);
         this.onLoadProc      = opts.getProc("on_load");
 
@@ -280,19 +290,23 @@ private ParserSession(ParserConfig config, RubyString source, ThreadContext cont
             this.decoder = new StringDecoder();
         }
 
-        private RaiseException unexpectedToken(ThreadContext context, int absStart, int absEnd) {
+        private RaiseException parsingError(ThreadContext context, String message, int absStart, int absEnd) {
             RubyString msg = context.runtime.newString("unexpected token at '")
                     .cat(data, absStart, Math.min(absEnd - absStart, 32))
                     .cat((byte)'\'');
             return newException(context, Utils.M_PARSER_ERROR, msg);
         }
 
+        private RaiseException unexpectedToken(ThreadContext context, int absStart, int absEnd) {
+            return parsingError(context, "unexpected token at '", absStart, absEnd);
+        }
+
         
-// line 314 "ParserConfig.rl"
+// line 328 "ParserConfig.rl"
 
 
         
-// line 296 "ParserConfig.java"
+// line 310 "ParserConfig.java"
 private static byte[] init__JSON_value_actions_0()
 {
 	return new byte [] {
@@ -406,7 +420,7 @@ private static byte[] init__JSON_value_from_state_actions_0()
 static final int JSON_value_en_main = 1;
 
 
-// line 420 "ParserConfig.rl"
+// line 434 "ParserConfig.rl"
 
 
         void parseValue(ThreadContext context, ParserResult res, int p, int pe) {
@@ -414,14 +428,14 @@ void parseValue(ThreadContext context, ParserResult res, int p, int pe) {
             IRubyObject result = null;
 
             
-// line 418 "ParserConfig.java"
+// line 432 "ParserConfig.java"
 	{
 	cs = JSON_value_start;
 	}
 
-// line 427 "ParserConfig.rl"
+// line 441 "ParserConfig.rl"
             
-// line 425 "ParserConfig.java"
+// line 439 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -447,13 +461,13 @@ void parseValue(ThreadContext context, ParserResult res, int p, int pe) {
 	while ( _nacts-- > 0 ) {
 		switch ( _JSON_value_actions[_acts++] ) {
 	case 9:
-// line 405 "ParserConfig.rl"
+// line 419 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 457 "ParserConfig.java"
+// line 471 "ParserConfig.java"
 		}
 	}
 
@@ -516,25 +530,25 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
 			switch ( _JSON_value_actions[_acts++] )
 			{
 	case 0:
-// line 322 "ParserConfig.rl"
+// line 336 "ParserConfig.rl"
 	{
                 result = context.nil;
             }
 	break;
 	case 1:
-// line 325 "ParserConfig.rl"
+// line 339 "ParserConfig.rl"
 	{
                 result = context.fals;
             }
 	break;
 	case 2:
-// line 328 "ParserConfig.rl"
+// line 342 "ParserConfig.rl"
 	{
                 result = context.tru;
             }
 	break;
 	case 3:
-// line 331 "ParserConfig.rl"
+// line 345 "ParserConfig.rl"
 	{
                 if (config.allowNaN) {
                     result = getConstant(CONST_NAN);
@@ -544,7 +558,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
             }
 	break;
 	case 4:
-// line 338 "ParserConfig.rl"
+// line 352 "ParserConfig.rl"
 	{
                 if (config.allowNaN) {
                     result = getConstant(CONST_INFINITY);
@@ -554,7 +568,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
             }
 	break;
 	case 5:
-// line 345 "ParserConfig.rl"
+// line 359 "ParserConfig.rl"
 	{
                 if (pe > p + 8 &&
                     absSubSequence(p, p + 9).equals(JSON_MINUS_INFINITY)) {
@@ -583,7 +597,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
             }
 	break;
 	case 6:
-// line 371 "ParserConfig.rl"
+// line 385 "ParserConfig.rl"
 	{
                 parseString(context, res, p, pe);
                 if (res.result == null) {
@@ -596,7 +610,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
             }
 	break;
 	case 7:
-// line 381 "ParserConfig.rl"
+// line 395 "ParserConfig.rl"
 	{
                 currentNesting++;
                 parseArray(context, res, p, pe);
@@ -611,7 +625,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
             }
 	break;
 	case 8:
-// line 393 "ParserConfig.rl"
+// line 407 "ParserConfig.rl"
 	{
                 currentNesting++;
                 parseObject(context, res, p, pe);
@@ -625,7 +639,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
                 }
             }
 	break;
-// line 629 "ParserConfig.java"
+// line 643 "ParserConfig.java"
 			}
 		}
 	}
@@ -645,7 +659,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 428 "ParserConfig.rl"
+// line 442 "ParserConfig.rl"
 
             if (cs >= JSON_value_first_final && result != null) {
                 if (config.freeze) {
@@ -658,7 +672,7 @@ else if ( data[p] > _JSON_value_trans_keys[_mid+1] )
         }
 
         
-// line 662 "ParserConfig.java"
+// line 676 "ParserConfig.java"
 private static byte[] init__JSON_integer_actions_0()
 {
 	return new byte [] {
@@ -757,7 +771,7 @@ private static byte[] init__JSON_integer_trans_actions_0()
 static final int JSON_integer_en_main = 1;
 
 
-// line 450 "ParserConfig.rl"
+// line 464 "ParserConfig.rl"
 
 
         void parseInteger(ThreadContext context, ParserResult res, int p, int pe) {
@@ -774,15 +788,15 @@ int parseIntegerInternal(int p, int pe) {
             int cs;
 
             
-// line 778 "ParserConfig.java"
+// line 792 "ParserConfig.java"
 	{
 	cs = JSON_integer_start;
 	}
 
-// line 466 "ParserConfig.rl"
+// line 480 "ParserConfig.rl"
             int memo = p;
             
-// line 786 "ParserConfig.java"
+// line 800 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -863,13 +877,13 @@ else if ( data[p] > _JSON_integer_trans_keys[_mid+1] )
 			switch ( _JSON_integer_actions[_acts++] )
 			{
 	case 0:
-// line 444 "ParserConfig.rl"
+// line 458 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 873 "ParserConfig.java"
+// line 887 "ParserConfig.java"
 			}
 		}
 	}
@@ -889,7 +903,7 @@ else if ( data[p] > _JSON_integer_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 468 "ParserConfig.rl"
+// line 482 "ParserConfig.rl"
 
             if (cs < JSON_integer_first_final) {
                 return -1;
@@ -909,7 +923,7 @@ RubyInteger bytesToInum(Ruby runtime, ByteList num) {
         }
 
         
-// line 913 "ParserConfig.java"
+// line 927 "ParserConfig.java"
 private static byte[] init__JSON_float_actions_0()
 {
 	return new byte [] {
@@ -1011,7 +1025,7 @@ private static byte[] init__JSON_float_trans_actions_0()
 static final int JSON_float_en_main = 1;
 
 
-// line 501 "ParserConfig.rl"
+// line 515 "ParserConfig.rl"
 
 
         void parseFloat(ThreadContext context, ParserResult res, int p, int pe) {
@@ -1030,15 +1044,15 @@ int parseFloatInternal(int p, int pe) {
             int cs;
 
             
-// line 1034 "ParserConfig.java"
+// line 1048 "ParserConfig.java"
 	{
 	cs = JSON_float_start;
 	}
 
-// line 519 "ParserConfig.rl"
+// line 533 "ParserConfig.rl"
             int memo = p;
             
-// line 1042 "ParserConfig.java"
+// line 1056 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -1119,13 +1133,13 @@ else if ( data[p] > _JSON_float_trans_keys[_mid+1] )
 			switch ( _JSON_float_actions[_acts++] )
 			{
 	case 0:
-// line 492 "ParserConfig.rl"
+// line 506 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 1129 "ParserConfig.java"
+// line 1143 "ParserConfig.java"
 			}
 		}
 	}
@@ -1145,7 +1159,7 @@ else if ( data[p] > _JSON_float_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 521 "ParserConfig.rl"
+// line 535 "ParserConfig.rl"
 
             if (cs < JSON_float_first_final) {
                 return -1;
@@ -1155,7 +1169,7 @@ else if ( data[p] > _JSON_float_trans_keys[_mid+1] )
         }
 
         
-// line 1159 "ParserConfig.java"
+// line 1173 "ParserConfig.java"
 private static byte[] init__JSON_string_actions_0()
 {
 	return new byte [] {
@@ -1257,7 +1271,7 @@ private static byte[] init__JSON_string_trans_actions_0()
 static final int JSON_string_en_main = 1;
 
 
-// line 560 "ParserConfig.rl"
+// line 574 "ParserConfig.rl"
 
 
         void parseString(ThreadContext context, ParserResult res, int p, int pe) {
@@ -1265,15 +1279,15 @@ void parseString(ThreadContext context, ParserResult res, int p, int pe) {
             IRubyObject result = null;
 
             
-// line 1269 "ParserConfig.java"
+// line 1283 "ParserConfig.java"
 	{
 	cs = JSON_string_start;
 	}
 
-// line 567 "ParserConfig.rl"
+// line 581 "ParserConfig.rl"
             int memo = p;
             
-// line 1277 "ParserConfig.java"
+// line 1291 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -1354,7 +1368,7 @@ else if ( data[p] > _JSON_string_trans_keys[_mid+1] )
 			switch ( _JSON_string_actions[_acts++] )
 			{
 	case 0:
-// line 535 "ParserConfig.rl"
+// line 549 "ParserConfig.rl"
 	{
                 int offset = byteList.begin();
                 ByteList decoded = decoder.decode(context, byteList, memo + 1 - offset,
@@ -1369,13 +1383,13 @@ else if ( data[p] > _JSON_string_trans_keys[_mid+1] )
             }
 	break;
 	case 1:
-// line 548 "ParserConfig.rl"
+// line 562 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 1379 "ParserConfig.java"
+// line 1393 "ParserConfig.java"
 			}
 		}
 	}
@@ -1395,7 +1409,7 @@ else if ( data[p] > _JSON_string_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 569 "ParserConfig.rl"
+// line 583 "ParserConfig.rl"
 
             if (cs >= JSON_string_first_final && result != null) {
                 if (result instanceof RubyString) {
@@ -1416,7 +1430,7 @@ else if ( data[p] > _JSON_string_trans_keys[_mid+1] )
         }
 
         
-// line 1420 "ParserConfig.java"
+// line 1434 "ParserConfig.java"
 private static byte[] init__JSON_array_actions_0()
 {
 	return new byte [] {
@@ -1583,7 +1597,7 @@ private static byte[] init__JSON_array_trans_actions_0()
 static final int JSON_array_en_main = 1;
 
 
-// line 623 "ParserConfig.rl"
+// line 637 "ParserConfig.rl"
 
 
         void parseArray(ThreadContext context, ParserResult res, int p, int pe) {
@@ -1597,14 +1611,14 @@ void parseArray(ThreadContext context, ParserResult res, int p, int pe) {
             IRubyObject result = RubyArray.newArray(context.runtime);
 
             
-// line 1601 "ParserConfig.java"
+// line 1615 "ParserConfig.java"
 	{
 	cs = JSON_array_start;
 	}
 
-// line 636 "ParserConfig.rl"
+// line 650 "ParserConfig.rl"
             
-// line 1608 "ParserConfig.java"
+// line 1622 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -1647,7 +1661,7 @@ else if ( _widec > _JSON_array_cond_keys[_mid+1] )
 	case 0: {
 		_widec = 65536 + (data[p] - 0);
 		if ( 
-// line 594 "ParserConfig.rl"
+// line 608 "ParserConfig.rl"
  config.allowTrailingComma  ) _widec += 65536;
 		break;
 	}
@@ -1717,7 +1731,7 @@ else if ( _widec > _JSON_array_trans_keys[_mid+1] )
 			switch ( _JSON_array_actions[_acts++] )
 			{
 	case 0:
-// line 596 "ParserConfig.rl"
+// line 610 "ParserConfig.rl"
 	{
                 parseValue(context, res, p, pe);
                 if (res.result == null) {
@@ -1730,13 +1744,13 @@ else if ( _widec > _JSON_array_trans_keys[_mid+1] )
             }
 	break;
 	case 1:
-// line 607 "ParserConfig.rl"
+// line 621 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 1740 "ParserConfig.java"
+// line 1754 "ParserConfig.java"
 			}
 		}
 	}
@@ -1756,7 +1770,7 @@ else if ( _widec > _JSON_array_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 637 "ParserConfig.rl"
+// line 651 "ParserConfig.rl"
 
             if (cs >= JSON_array_first_final) {
                 res.update(config.onLoad(context, result), p + 1);
@@ -1766,7 +1780,7 @@ else if ( _widec > _JSON_array_trans_keys[_mid+1] )
         }
 
         
-// line 1770 "ParserConfig.java"
+// line 1784 "ParserConfig.java"
 private static byte[] init__JSON_object_actions_0()
 {
 	return new byte [] {
@@ -1943,7 +1957,7 @@ private static byte[] init__JSON_object_trans_actions_0()
 static final int JSON_object_en_main = 1;
 
 
-// line 694 "ParserConfig.rl"
+// line 721 "ParserConfig.rl"
 
 
         void parseObject(ThreadContext context, ParserResult res, int p, int pe) {
@@ -1960,14 +1974,14 @@ void parseObject(ThreadContext context, ParserResult res, int p, int pe) {
             IRubyObject result = RubyHash.newHash(context.runtime);
 
             
-// line 1964 "ParserConfig.java"
+// line 1978 "ParserConfig.java"
 	{
 	cs = JSON_object_start;
 	}
 
-// line 710 "ParserConfig.rl"
+// line 737 "ParserConfig.rl"
             
-// line 1971 "ParserConfig.java"
+// line 1985 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -2010,7 +2024,7 @@ else if ( _widec > _JSON_object_cond_keys[_mid+1] )
 	case 0: {
 		_widec = 65536 + (data[p] - 0);
 		if ( 
-// line 651 "ParserConfig.rl"
+// line 665 "ParserConfig.rl"
  config.allowTrailingComma  ) _widec += 65536;
 		break;
 	}
@@ -2080,7 +2094,7 @@ else if ( _widec > _JSON_object_trans_keys[_mid+1] )
 			switch ( _JSON_object_actions[_acts++] )
 			{
 	case 0:
-// line 653 "ParserConfig.rl"
+// line 667 "ParserConfig.rl"
 	{
                 parseValue(context, res, p, pe);
                 if (res.result == null) {
@@ -2093,7 +2107,7 @@ else if ( _widec > _JSON_object_trans_keys[_mid+1] )
             }
 	break;
 	case 1:
-// line 664 "ParserConfig.rl"
+// line 678 "ParserConfig.rl"
 	{
                 parseString(context, res, p, pe);
                 if (res.result == null) {
@@ -2106,18 +2120,31 @@ else if ( _widec > _JSON_object_trans_keys[_mid+1] )
                     } else {
                         lastName = name;
                     }
+
+                    if (!config.allowDuplicateKey) {
+                        if (((RubyHash)result).hasKey(lastName)) {
+                            if (config.deprecateDuplicateKey) {
+                                context.runtime.getWarnings().warning(
+                                    "detected duplicate keys in JSON object. This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`"
+                                );
+                            } else {
+                                throw parsingError(context, "duplicate key", p, pe);
+                            }
+                        }
+                    }
+
                     {p = (( res.p))-1;}
                 }
             }
 	break;
 	case 2:
-// line 680 "ParserConfig.rl"
+// line 707 "ParserConfig.rl"
 	{
                 p--;
                 { p += 1; _goto_targ = 5; if (true)  continue _goto;}
             }
 	break;
-// line 2121 "ParserConfig.java"
+// line 2148 "ParserConfig.java"
 			}
 		}
 	}
@@ -2137,7 +2164,7 @@ else if ( _widec > _JSON_object_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 711 "ParserConfig.rl"
+// line 738 "ParserConfig.rl"
 
             if (cs < JSON_object_first_final) {
                 res.update(null, p + 1);
@@ -2148,7 +2175,7 @@ else if ( _widec > _JSON_object_trans_keys[_mid+1] )
         }
 
         
-// line 2152 "ParserConfig.java"
+// line 2179 "ParserConfig.java"
 private static byte[] init__JSON_actions_0()
 {
 	return new byte [] {
@@ -2251,7 +2278,7 @@ private static byte[] init__JSON_trans_actions_0()
 static final int JSON_en_main = 1;
 
 
-// line 740 "ParserConfig.rl"
+// line 767 "ParserConfig.rl"
 
 
         public IRubyObject parseImplementation(ThreadContext context) {
@@ -2261,16 +2288,16 @@ public IRubyObject parseImplementation(ThreadContext context) {
             ParserResult res = new ParserResult();
 
             
-// line 2265 "ParserConfig.java"
+// line 2292 "ParserConfig.java"
 	{
 	cs = JSON_start;
 	}
 
-// line 749 "ParserConfig.rl"
+// line 776 "ParserConfig.rl"
             p = byteList.begin();
             pe = p + byteList.length();
             
-// line 2274 "ParserConfig.java"
+// line 2301 "ParserConfig.java"
 	{
 	int _klen;
 	int _trans = 0;
@@ -2351,7 +2378,7 @@ else if ( data[p] > _JSON_trans_keys[_mid+1] )
 			switch ( _JSON_actions[_acts++] )
 			{
 	case 0:
-// line 726 "ParserConfig.rl"
+// line 753 "ParserConfig.rl"
 	{
                 parseValue(context, res, p, pe);
                 if (res.result == null) {
@@ -2363,7 +2390,7 @@ else if ( data[p] > _JSON_trans_keys[_mid+1] )
                 }
             }
 	break;
-// line 2367 "ParserConfig.java"
+// line 2394 "ParserConfig.java"
 			}
 		}
 	}
@@ -2383,7 +2410,7 @@ else if ( data[p] > _JSON_trans_keys[_mid+1] )
 	break; }
 	}
 
-// line 752 "ParserConfig.rl"
+// line 779 "ParserConfig.rl"
 
             if (cs >= JSON_first_final && p == pe) {
                 return result;
diff --git a/java/src/json/ext/ParserConfig.rl b/java/src/json/ext/ParserConfig.rl
index ce0d9438e..4bc5d93bd 100644
--- a/java/src/json/ext/ParserConfig.rl
+++ b/java/src/json/ext/ParserConfig.rl
@@ -52,6 +52,8 @@ public class ParserConfig extends RubyObject {
     private int maxNesting;
     private boolean allowNaN;
     private boolean allowTrailingComma;
+    private boolean allowDuplicateKey;
+    private boolean deprecateDuplicateKey;
     private boolean symbolizeNames;
     private boolean freeze;
     private RubyProc onLoadProc;
@@ -175,6 +177,14 @@ public class ParserConfig extends RubyObject {
         this.allowNaN        = opts.getBool("allow_nan", false);
         this.allowTrailingComma = opts.getBool("allow_trailing_comma", false);
         this.symbolizeNames  = opts.getBool("symbolize_names", false);
+        if (opts.hasKey("allow_duplicate_key")) {
+            this.allowDuplicateKey = opts.getBool("allow_duplicate_key", false);
+            this.deprecateDuplicateKey = false;
+        } else {
+            this.allowDuplicateKey = false;
+            this.deprecateDuplicateKey = true;
+        }
+
         this.freeze          = opts.getBool("freeze", false);
         this.onLoadProc      = opts.getProc("on_load");
 
@@ -278,13 +288,17 @@ public class ParserConfig extends RubyObject {
             this.decoder = new StringDecoder();
         }
 
-        private RaiseException unexpectedToken(ThreadContext context, int absStart, int absEnd) {
+        private RaiseException parsingError(ThreadContext context, String message, int absStart, int absEnd) {
             RubyString msg = context.runtime.newString("unexpected token at '")
                     .cat(data, absStart, Math.min(absEnd - absStart, 32))
                     .cat((byte)'\'');
             return newException(context, Utils.M_PARSER_ERROR, msg);
         }
 
+        private RaiseException unexpectedToken(ThreadContext context, int absStart, int absEnd) {
+            return parsingError(context, "unexpected token at '", absStart, absEnd);
+        }
+
         %%{
             machine JSON_common;
 
@@ -673,6 +687,19 @@ public class ParserConfig extends RubyObject {
                     } else {
                         lastName = name;
                     }
+
+                    if (!config.allowDuplicateKey) {
+                        if (((RubyHash)result).hasKey(lastName)) {
+                            if (config.deprecateDuplicateKey) {
+                                context.runtime.getWarnings().warning(
+                                    "detected duplicate keys in JSON object. This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`"
+                                );
+                            } else {
+                                throw parsingError(context, "duplicate key", p, pe);
+                            }
+                        }
+                    }
+
                     fexec res.p;
                 }
             }
diff --git a/lib/json.rb b/lib/json.rb
index 264301e05..735f23806 100644
--- a/lib/json.rb
+++ b/lib/json.rb
@@ -127,6 +127,24 @@
 #
 # ---
 #
+# Option +allow_duplicate_key+ specifies whether duplicate keys in objects
+# should be ignored or cause an error to be raised:
+#
+# When not specified:
+#   # The last value is used and a deprecation warning emitted.
+#   JSON.parse('{"a": 1, "a":2}') => {"a" => 2}
+#   # waring: detected duplicate keys in JSON object.
+#   # This will raise an error in json 3.0 unless enabled via `allow_duplicate_key: true`
+#
+# When set to `+true+`
+#   # The last value is used.
+#   JSON.parse('{"a": 1, "a":2}') => {"a" => 2}
+#
+# When set to `+false+`, the future default:
+#   JSON.parse('{"a": 1, "a":2}') => duplicate key at line 1 column 1 (JSON::ParserError)
+#
+# ---
+#
 # Option +allow_nan+ (boolean) specifies whether to allow
 # NaN, Infinity, and MinusInfinity in +source+;
 # defaults to +false+.
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index befc80c95..739a4cf63 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -331,6 +331,15 @@ def test_parse_big_integers
     assert_equal orig, parse(json5)
   end
 
+  def test_parse_duplicate_key
+    expected = {"a" => 2}
+    assert_equal expected, parse('{"a": 1, "a": 2}', allow_duplicate_key: true)
+    assert_raise(ParserError) { parse('{"a": 1, "a": 2}', allow_duplicate_key: false) }
+    assert_deprecated_warning(/duplicate keys/) do
+      assert_equal expected, parse('{"a": 1, "a": 2}')
+    end
+  end
+
   def test_some_wrong_inputs
     assert_raise(ParserError) { parse('[] bla') }
     assert_raise(ParserError) { parse('[] 1') }