Fixes #1825 -- document that multiple adds of the same model is a no-op.

index.html

@@ -1558,6 +1558,11 @@ ships.add([
 ]);
 </pre>
 
+    <p class="warning">
+      Note that adding the same model (a model with the same <tt>id</tt>) to
+      a collection more than once <br /> is a no-op.
+    </p>
+
     <p id="Collection-remove">
       <b class="header">remove</b><code>collection.remove(models, [options])</code>
       <br />
@@ -3213,18 +3218,18 @@ var model = localBackbone.Model.extend(...);
         <img width="550" height="435" data-original="docs/images/battlefield.png" alt="Battlefield Play4Free" class="example_image" />
       </a>
     </div>
-    
+
     <h2 id="examples-syllabus">Syllabus</h2>
 
     <p>
       <a href="http://product.voxmedia.com/post/25113965826/introducing-syllabus-vox-medias-s3-powered-liveblog">Syllabus</a>
-      is the new live blogging platform used by 
-      <a href="http://www.theverge.com/">The Verge</a> 
+      is the new live blogging platform used by
+      <a href="http://www.theverge.com/">The Verge</a>
       and other <a href="http://www.voxmedia.com/">Vox Media</a> sites.
-      Syllabus uses Backbone on both ends: an editorial dashboard and the live blog 
-      page itself. In the back, Backbone is used to provide a 
-      single-page experience for uploading, writing, editing and publishing content. 
-      On the live blog, Backbone manages fetching a JSON API feed, and updating 
+      Syllabus uses Backbone on both ends: an editorial dashboard and the live blog
+      page itself. In the back, Backbone is used to provide a
+      single-page experience for uploading, writing, editing and publishing content.
+      On the live blog, Backbone manages fetching a JSON API feed, and updating
       the infinite-scrolling river of updates with new and revised content.
     </p>