title: Identify duplicate values in a list in Python
body: <p>Is it possible to get which values are duplicates in a list using python?</p>
  
  <p>I have a list of items:</p>
  
  <pre><code>    mylist = [20, 30, 25, 20]
  </code></pre>
  
  <p>I know the best way of removing the duplicates is <code>set(mylist)</code>, but is it possible to know what values are being duplicated? As you can see, in this list the duplicates are the first and last values. <code>[0, 3]</code>.</p>
  
  <p>Is it possible to get this result or something similar in python? I'm trying to avoid making a ridiculously big <code>if elif</code> conditional statement.</p>