title: Ember Data has duplicate records
body: <p>In my app, a user can create a message and send it. When the user sends the message, the message is created with <code>createRecord</code> and the server replies with 201 Created if successful.</p>
  
  <p>Also, the user can get messages from other users through a websocket. When it receives a message, I push it into the store with <code>pushPayload</code>.</p>
  
  <pre><code>var parsedData = JSON.parse(data);
  this.store.pushPayload('message', parsedData);
  </code></pre>
  
  <p>The problem is, when a user sends a message and saves it, they also get it back from the websocket, and even though both objects have the same id, the store ends up with duplicate messages.</p>
  
  <p>How can I tell the store than when I push or save something with the same id of an already existing element, it should override it?</p>