title: How to return a list containing common elements with no duplicates
body: <pre><code>def common_elements(list1, list2):
      """
      Return a list containing the elements which are in both list1 and list2
  
      &gt;&gt;&gt; common_elements([1,2,3,4,5,6], [3,5,7,9])
      [3, 5]
      &gt;&gt;&gt; common_elements(["this","this","n","that"],["this","not","that","that"])
      ['this', 'that']
      """
  
      result = []
      for element in list1:
          if element in list2:
              result.append(element)
      return result
  </code></pre>
  
  <p>I have this so far but it returns with duplicates for example:</p>
  
  <pre><code>common_elements(["this","this","n","that"],["this","not","that","that"])
  </code></pre>
  
  <p>Returns as: <code>['this', 'this', 'that']</code></p>