title: find duplicate rows in a pandas dataframe
body: <p>I am trying to find duplicates rows in a pandas dataframe.</p>
  
  <pre><code>df=pd.DataFrame(data=[[1,2],[3,4],[1,2],[1,4],[1,2]],columns=['col1','col2'])
  
  df
  Out[15]: 
     col1  col2
  0     1     2
  1     3     4
  2     1     2
  3     1     4
  4     1     2
  
  duplicate_bool = df.duplicated(subset=['col1','col2'], keep='first')
  duplicate = df.loc[duplicate_bool == True]
  
  duplicate
  Out[16]: 
     col1  col2
  2     1     2
  4     1     2
  </code></pre>
  
  <p>Is there a way to add a column referring to the index of the first duplicate (the one kept)</p>
  
  <pre><code>duplicate
  Out[16]: 
     col1  col2  index_original
  2     1     2               0
  4     1     2               0
  </code></pre>
  
  <p>Note: df could be very very big in my case....</p>