title: Backbone.js updating of models in a collection
body: <p>Let's say you are building a Twitter clone with Backbone.js. You have a collection of tweets. Each tweet is obviously an instance of the Tweet model.</p>
  
  <p>You create an instance of the collection, fetch the latest 10 tweets, render them and add to the DOM. </p>
  
  <p>So far so good.</p>
  
  <p>What if you wanted to make a call to the server a few minutes later to see if any new tweets arrived? How can you add the newly arrived tweets to the collection?</p>
  
  <p>If you use the <code>fetch()</code> method, you are hitting the same URL all the time. That's fine. Is there a clever way that I can use Backbone/Underscore to filter those and add the tweets that aren't in the collection to the collection?</p>