Normative: Remove proxy OwnPropertyKeys error with duplicate keys (#594)

Previously, proxy OwnPropertyKeys returning duplicate keys would
cause an error when the target object was non-extensible. The error
does not seem deliberate but rather a result of not considering the
possibility of duplicate keys. This PR handles that possibility and
removes the error.

spec.html

@@ -8687,11 +8687,11 @@ <h1>[[OwnPropertyKeys]] ( )</h1>
         1. Let _uncheckedResultKeys_ be a new List which is a copy of _trapResult_.
         1. Repeat, for each _key_ that is an element of _targetNonconfigurableKeys_,
           1. If _key_ is not an element of _uncheckedResultKeys_, throw a *TypeError* exception.
-          1. Remove _key_ from _uncheckedResultKeys_.
+          1. Remove all occurrences of _key_ from _uncheckedResultKeys_.
         1. If _extensibleTarget_ is *true*, return _trapResult_.
         1. Repeat, for each _key_ that is an element of _targetConfigurableKeys_,
           1. If _key_ is not an element of _uncheckedResultKeys_, throw a *TypeError* exception.
-          1. Remove _key_ from _uncheckedResultKeys_.
+          1. Remove all occurrences of _key_ from _uncheckedResultKeys_.
         1. If _uncheckedResultKeys_ is not empty, throw a *TypeError* exception.
         1. Return _trapResult_.
       </emu-alg>