title: How to find duplicate items in list<>?
body: <p>I have: </p>
  
  <pre><code>List&lt;string&gt; list = new List&lt;string&gt;() { "a", "a", "b", "b", "r", "t" };
  </code></pre>
  
  <p>How can I get only "a","b"?</p>
  
  <p>I tried to do like this:</p>
  
  <pre><code>List&lt;string&gt; list = new List&lt;string&gt;() { "a", "a", "b", "b", "r", "t" };
  List&lt;string&gt; test_list = new List&lt;string&gt;(); 
  
  test_list = list.Distinct().ToList();
  </code></pre>
  
  <p>Now test_list has {"a", "b", "r", "t"}<br>
  And then:</p>
  
  <pre><code>test_list = test_list.Except(list).ToList();
  </code></pre>
  
  <p>So that was my fail point, cause Except() deleted all elements.</p>
  
  <p><em>Could you help me with solution?</em></p>