add dup __proto__ in json test

test/built-ins/JSON/parse/duplicate-proto.js

@@ -0,0 +1,17 @@
+// Copyright (C) 2020 devsnek. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-object-initializer-static-semantics-early-errors
+description: >
+  It is a Syntax Error if PropertyNameList of PropertyDefinitionList contains
+  any duplicate entries for "__proto__" and at least two of those entries were
+  obtained from productions of the form
+    PropertyDefinition : PropertyName `:` AssignmentExpression .
+  This rule is not applied if this PropertyDefinition is contained within a
+  Script which is being evaluated for JSON.parse (see step 4 of JSON.parse).
+---*/
+
+var result = JSON.parse('{ "__proto__": 1, "__proto__": 2 }');
+
+assert.sameValue(result.__proto__, 2);