title: Add elements from a list to another one if they aren't already contained
body: <p>I have 2 lists, I need to add the elements contained in the first list to the second list.
  If the second list contain some of the element of the first list, those shoudn't be added to the second list (to avoid repetitions).</p>
  
  <p>At the moment I'm using:</p>
  
  <pre><code>set ListB to ListB &amp; ListA
  </code></pre>
  
  <p>But obviously these doesn't take into account duplicates.</p>