title: How to find duplicates in list and sort them by number of appearance
body: <p>Is there a easy way to find duplicates in list, and then sort them by number of appearance? Also, duplicates should be removed.</p>
  
  <p>Eg. you have a <code>List&lt;String&gt;</code> like this:</p>
  
  <pre><code>List&lt;String&gt; = new List&lt;String&gt;{"6","1","2","2","4","6","5","1","6","6","2"};
  </code></pre>
  
  <p>Question is, how to convert this list into -> "6", "1", "2", "4", "5"?</p>