Add a note about duplicate event listeners for removeEventListener()

Fixes #664.

Author: LukeZielinski

dom.bs

@@ -1216,6 +1216,10 @@ method, when invoked, must run these steps:
  with the <a>context object</a> and that <a>event listener</a>.
 </ol>
 
+<p class=note>The event listener list will not contain multiple event listeners with equal
+<var>type</var>, <var>callback</var>, and <var>capture</var>, as <a>add an event listener</a>
+prevents that.
+
 <p>The <dfn method for=EventTarget><code>dispatchEvent(<var>event</var>)</code></dfn> method, when
 invoked, must run these steps:
 
@@ -10037,6 +10041,7 @@ Kirill Topolyan,
 Koji Ishii,
 Lachlan Hunt,
 Lauren Wood,
+Luke Zielinski,
 Magne Andersson,
 Majid Valipour,
 Malte Ubl,