ensure uniqueness for addListener

Author: annevk

cssom-view/Overview.html

@@ -546,15 +546,16 @@ <h3 id=the-mediaquerylist-interface><span class=secno>3.1. </span>The <code
    title="">listener</var>)</code></dfn> is invoked <var
    title="">listener</var> <em class=ct>must</em> be appended to the <a
    href="#list-of-media-query-list-listeners">list of media query list
+   listeners</a>, unless it is already in the <a
+   href="#list-of-media-query-list-listeners">list of media query list
    listeners</a>.
 
   <p>When the <dfn id=dom-mediaquerylist-removelistener
    title=dom-MediaQueryList-removeListener><code>removeListener(<var
-   title="">listener</var>)</code></dfn> is invoked the last listener in the
-   <a href="#list-of-media-query-list-listeners">list of media query list
-   listeners</a> that matches <var title="">listener</var> <e class=ct>must
-   be removed from the <a href="#list-of-media-query-list-listeners">list of
-   media query list listeners</a>.</e>
+   title="">listener</var>)</code></dfn> is invoked <var
+   title="">listener</var> <e class=ct>must be removed from the <a
+   href="#list-of-media-query-list-listeners">list of media query list
+   listeners</a>.</e>
 
   <h3 id=the-screen-interface><span class=secno>3.2. </span>The <code
    title="">Screen</code> Interface</h3>

cssom-view/Overview.src.html

@@ -398,14 +398,13 @@ <h3 id="the-mediaquerylist-interface">The <code title="">MediaQueryList</code> I
   <p>When the
   <dfn id="dom-mediaquerylist-addlistener" title="dom-MediaQueryList-addListener"><code>addListener(<var title="">listener</var>)</code></dfn>
   is invoked <var title="">listener</var> <em class="ct">must</em> be
-  appended to the <span>list of media query list listeners</span>.</p>
+  appended to the <span>list of media query list listeners</span>, unless
+  it is already in the <span>list of media query list listeners</span>.</p>
 
   <p>When the
   <dfn id="dom-mediaquerylist-removelistener" title="dom-MediaQueryList-removeListener"><code>removeListener(<var title="">listener</var>)</code></dfn>
-  is invoked the last listener in the
-  <span>list of media query list listeners</span> that matches
-  <var title="">listener</var> <e class="ct">must</em> be removed from the
-  <span>list of media query list listeners</span>.</p>
+  is invoked <var title="">listener</var> <e class="ct">must</em> be removed
+  from the <span>list of media query list listeners</span>.</p>