title: Find all duplicate columns in a collection of data frames
body: <p>Having a collection of data frames, the goal is to identify the duplicated column names and return them as a list.</p>
  
  <p>Example</p>
  
  <p>The input are 3 data frames <code>df1</code>, <code>df2</code> and <code>df3</code>:</p>
  
  <pre><code>df1 = pd.DataFrame({'a':[1,5], 'b':[3,9], 'e':[0,7]})
  
     a  b  e
  0  1  3  0
  1  5  9  7
  
  df2 = pd.DataFrame({'d':[2,3], 'e':[0,7], 'f':[2,1]})
  
     d  e  f
  0  2  0  2
  1  3  7  1
  
  df3 = pd.DataFrame({'b':[3,9], 'c':[8,2], 'e':[0,7]})
  
     b  c  e
  0  3  8  0
  1  9  2  7
  </code></pre>
  
  <p>The output is a list <code>[b, e]</code></p>