title: Find dictionary keys with duplicate values
body: <pre><code>some_dict = {"firstname": "Albert", "nickname": "Albert", "surname": "Likins", "username": "Angel"}
  </code></pre>
  
  <p>I would like to return the keys of my dict where their value exists for more than one time.</p>
  
  <p>Could some one show me how to implement this?</p>
  
  <pre><code>a_list = []
  for k,v in  some_dict.iteritems():
      if v in some_dict.values() and v != some_dict.keys(k):
          a_list.append(k)
  </code></pre>