Add back the duplicate property checker

test/test262/test262.status

 # Copyright 2011 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 59 matching lines @@
 
   'S15.8.2.8_A6': [PASS, FAIL_OK],  # Math.exp (less precise with --fast-math)
 
   # Linux for ia32 (and therefore simulators) default to extended 80 bit
   # floating point formats, so these tests checking 64-bit FP precision fail.
   # The other platforms/arch's pass these tests.
   # We follow the other major JS engines by keeping this default.
   'S8.5_A2.1': [PASS, FAIL_OK],
   'S8.5_A2.2': [PASS, FAIL_OK],
 
-  # ES6 allows duplicate properties
-  '11.1.5-4-4-a-1-s': [FAIL],
-  '11.1.5_4-4-b-1': [FAIL],
-  '11.1.5_4-4-b-2': [FAIL],
-  '11.1.5_4-4-c-1': [FAIL],
-  '11.1.5_4-4-c-2': [FAIL],
-  '11.1.5_4-4-d-1': [FAIL],
-  '11.1.5_4-4-d-2': [FAIL],
-  '11.1.5_4-4-d-3': [FAIL],
-  '11.1.5_4-4-d-4': [FAIL],
-
   ############################ INVALID TESTS #############################
 
   # The reference value calculated by Test262 is incorrect if you run these
   # tests in PST/PDT between first Sunday in March and first Sunday in April.
   # The DST switch was moved in 2007 whereas Test262 bases the reference value
   # on 2000. Test262 Bug: https://bugs.ecmascript.org/show_bug.cgi?id=293
   'S15.9.3.1_A5_T1': [PASS, FAIL_OK],
   'S15.9.3.1_A5_T2': [PASS, FAIL_OK],
   'S15.9.3.1_A5_T3': [PASS, FAIL_OK],
   'S15.9.3.1_A5_T4': [PASS, FAIL_OK],
@@ skipping 20 matching lines @@
 
   # BUG(3251225): Tests that timeout with --nocrankshaft.
   'S15.1.3.1_A2.4_T1': [SKIP],
   'S15.1.3.1_A2.5_T1': [SKIP],
   'S15.1.3.2_A2.4_T1': [SKIP],
   'S15.1.3.2_A2.5_T1': [SKIP],
   'S15.1.3.3_A2.3_T1': [SKIP],
   'S15.1.3.4_A2.3_T1': [SKIP],
 }],  # 'arch == arm or arch == mipsel or arch == mips or arch == arm64'
 ]