title: How to double array values
body: <p>I have an array:</p>
  
  <pre><code>arr = ["a", "b", c"]
  </code></pre>
  
  <p>I want to double values like </p>
  
  <pre><code>arr = ["a", "b", c", "a", "b", "c"]
  </code></pre>
  
  <p>(not in particular order).</p>
  
  <p>What's the best way to do it in JavaScript?</p>