Proxy, [[OwnPropertyKeys]]: If trapResult contains any duplicate entr…

…ies, throw a TypeError

Ref: tc39/ecma262#833

test/built-ins/Proxy/ownKeys/return-duplicate-entries-throws.js

@@ -0,0 +1,22 @@
+// Copyright (C) 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-proxy-object-internal-methods-and-internal-slots-ownpropertykeys
+description: >
+    The returned list must not contain any duplicate entries.
+info: >
+    [[OwnPropertyKeys]] ( )
+
+    ...
+    9. If trapResult contains any duplicate entries, throw a TypeError exception.
+---*/
+
+var p = new Proxy({}, {
+  ownKeys() {
+    return ["a", "a"];
+  }
+});
+
+assert.throws(TypeError, function() {
+  Object.keys(p);
+});