title: Unable to drop duplicates in index in python DataFrame
body: <p>I am trying to drop duplicated index in multindexed python <code>pandas</code> <code>dataframe</code></p>
  
  <pre><code>    import pandas as pd
      arrays=[['i1', 'i2', 'i3'], ['j1', 'j2', 'j3']]
      tuples = list(zip(*arrays))
      index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
      df=pd.DataFrame({'a':['b', 'c', 'd']}, index=index)
      df = df.groupby(df.index).first()
      print(df)
  </code></pre>
  
  <p>but duplicates in index have not been removed:</p>
  
  <pre><code>          a
  (i1, j1)  b
  (i2, j2)  c
  (i3, j3)  d
  </code></pre>
  
  <p>How do I do that?</p>
  
  <p>Expected output:</p>
  
  <pre><code>          a
  (i1, j1)  b
  (i2, j2)  c
  
  </code></pre>