title: Add Range of items to list without duplication
body: <p>I have a <code>List</code> of <code>List</code> of <code>Strings</code>, and I need to use the <code>AddRange()</code> Function to add set of items to it, but never duplicate items.</p>
  
  <p>I used the following code :</p>
  
  <pre><code>List&lt;List&lt;string&gt;&gt; eList = new List&lt;List&lt;string&gt;&gt;();
  List&lt;List&lt;string&gt;&gt; mergedList = new List&lt;List&lt;string&gt;&gt;(); 
  
  //
  // some code here
  //
  
  mergedList.AddRange(eList.Where(x =&gt; !mergedList.Contains(x)).ToList());
  </code></pre>
  
  <p>However it does not work.</p>
  
  <p>All Duplicated items are added, so how could I solve that?</p>