title: Retrieve all value pairs from List<Integer>
body: <p>For example I have a <code>List&lt;Integer&gt;</code>  object with the following:</p>
  
  <pre><code>3, 6, 5, 3, 3, 6
  </code></pre>
  
  <p>The result would be 3 and 6. How can I create a function that tests for duplicates and then returns 1 value of the duplicate (not the pair, just one in the pair)? One problem that might occur is if there are quadruple values: <code>3, 4, 5, 3, 8, 3, 3</code> Then I would like to return 3 and 3. How can I accomplish this?</p>