title: Identify duplicates in a List
body: <p>I have a List of type Integer eg:</p>
  
  <pre><code>[1, 1, 2, 3, 3, 3]
  </code></pre>
  
  <p>I would like a method to return all the duplicates eg:</p>
  
  <pre><code>[1, 3]
  </code></pre>
  
  <p>What is the best way to do this?</p>