title: Find duplicates in a collection of lists
body: <p>If I have a dictionary containing 2 or more lists, how can I quickly find shared items between these lists and add these shared items to a list external to the dictionary?</p>
  
  <p>For example:</p>
  
  <p>list1:</p>
  
  <ul>
  <li>eng;English</li>
  <li>lir;Liberian English</li>
  </ul>
  
  <p>list2:</p>
  
  <ul>
  <li>eng;English</li>
  <li>bav;Vengo</li>
  </ul>
  
  <p>list3:</p>
  
  <ul>
  <li>lat;Latin</li>
  </ul>
  
  <p>extList:</p>
  
  <ul>
  <li>eng;English</li>
  </ul>
  
  <p>This shared item is then removed from the lists inside the dictionary.
  I have added <code>list3</code> to show that a superfluous item may be ignored, and that I have specified 2 or more lists.</p>