title: Finding the Location of the Duplicate for Duplicated Columns in Pandas
body: <p>I know I can find duplicate columns using:</p>
  
  <pre><code>df.T.duplicated()
  </code></pre>
  
  <p>what I'd like to know the index that a duplicate column is a duplicate of.  For example, both <code>C</code> and <code>D</code> are duplicates of a <code>A</code> below:</p>
  
  <pre><code>df = pd.DataFrame([[1,0,1,1], [2,0,2,2]], columns=['A', 'B', 'C', 'D'])
  
     A  B  C  D
  0  1  0  1  1
  1  2  0  2  2
  </code></pre>
  
  <p>I'd like something like:</p>
  
  <pre><code>duplicate_index = pd.Series([None, None, 'A', 'A'], ['A', 'B', 'C', 'D'])
  </code></pre>