title: Extract non unique values from array
body: <p>I can remove unique values from an array:</p>
  
  <pre><code>array_with_doubles = [[1, 2], [2, 3], [1, 2]]
  </code></pre>
  
  <p>using <code>uniq</code>:</p>
  
  <pre><code>array_with_doubles.uniq
  # =&gt; [[1, 2], [2, 3]]
  </code></pre>
  
  <p>I want to find which values were removed.</p>