title: Angular UI Select, Unique values pre-selected array
body: <p>Lets say I have want to edit an existing entity that has an array of values that are also part of the selectable values. e.g.</p>
  
  <pre><code>var preSelectedLanguages = [
      {id: 2, iso: "de"},
      {id: 3, iso: "fr"}
  ]
  
  var languages = [
      {id: 1, iso: "en"},
      {id: 2, iso: "de"},
      {id: 3, iso: "fr"},
      {id: 4, iso: "it"},
      {id: 5, iso: "us"}
  ]
  </code></pre>
  
  <p>My ui-select directive would be something like:</p>
  
  <pre><code>&lt;ui-select multiple ng-model="preSelectedLanguages"&gt;
      &lt;ui-select-match placeholder="Select language..."&gt;
          {{$item.iso}}
      &lt;/ui-select-match&gt;
      &lt;ui-select-choices repeat="l in languages track by language.id"&gt;
          {{language.iso}}
      &lt;/ui-select-choices&gt;
  &lt;/ui-select&gt;
  </code></pre>
  
  <p>However the <code>ui-select-choices</code> option list cant seem to remove the duplicates, even though I have used <code>track by language.id</code>.</p>
  
  <p>Any idea how to do this correctly?</p>