Duplicates in StructuredSerializeWithTransfer's transferList

Duplicates are not allowed in most implementations and throw an error (before any serialization exceptions). This aligns the standard with that behavior.

Tests: web-platform-tests/wpt#9672.

Fixes whatwg#3507.

Author: annevk

source

@@ -8813,12 +8813,15 @@ o.myself = o;</pre>
      <var>transferable</var>.<span>[[Detached]]</span> is true, then throw a
      <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
 
+     <li><p>If <var>memory</var>[<var>transferable</var>] <span data-x="map exists">exists</span>,
+     then throw a <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
+
      <li>
       <p><span data-x="map set">Set</span> <var>memory</var>[<var>transferable</var>] to {
       [[Type]]: an uninitialized value }.</p>
 
-      <p class="note"><var>transferable</var> is not transferred yet as that operation has side
-      effects and <span>StructuredSerializeInternal</span> can throw.</p>
+      <p class="note"><var>transferable</var> is not transferred yet as transferring has side
+      effects and <span>StructuredSerializeInternal</span> needs to be able to throw first.</p>
      </li>
     </ol>
    </li>