title: Optimal Way to Remove Unique Values from Two Arrays
body: <p>I have two arrays of [PFObjects]. </p>
  
  <p>For example (simplified):</p>
  
  <pre><code>arr1: [PFObject] = [1, 2, 3, 4, 5, 6, 7, 8]
  
  arr2: [PFObject] = [1, 2, 3, 4, 5]
  </code></pre>
  
  <p>What is the optimal way to compare <code>arr1</code> with <code>arr2</code> and only keep the duplicates (remove unique values).</p>
  
  <p>So that <code>arr1</code> looks like:</p>
  
  <pre><code>arr1 = [1, 2, 3, 4, 5]
  </code></pre>