Disallow registering the same constructor twice

spec/custom/index.html

@@ -364,7 +364,9 @@ <h3>New <a href="https://dom.spec.whatwg.org/#document"><code>Document</code></a
 
     <li>Let <var>registry</var> be the <a href="https://dom.spec.whatwg.org/#context-object">context object</a>'s <a>registry</a>. If <var>registry</var> does not exist, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>
 
-    <li>If <var>registry</var> contains an entry with <a href="#dfn-element-definition-type">custom element type</a> <var>type</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>
+    <li>If <var>registry</var> contains an entry with <a href="#dfn-element-definition-type">type</a> <var>type</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>
+
+    <li>If <var>registry</var> contains an entry with <a href="#dfn-element-definition-constructor">contructor</a> <var>constructor</var>, <a href="https://heycam.github.io/webidl/#dfn-throw">throw</a> a <code>NotSupportedError</code> and abort these steps.</li>
 
     <li>Let <var>localName</var> be <var>type</var>.</li>