title: ValueError: "cannot reindex from a duplicate axis" in groupby Pandas
body: <p>My dataframe looks like this:</p>
  
  <pre>
      SKU #    GRP    CATG   PRD
  0   54995  9404000  4040  99999
  1   54999  9404000  4040  99999
  2   55037  9404000  4040  1556894
  3   55148  9404000  4040  1556894
  4   55254  9404000  4040  1556894
  5   55291  9404000  4040  1556894
  6   55294  9404000  4040  1556895
  7   55445  9404000  4040  1556895
  8   55807  9404001  4040  1556896
  9   49021  9404002  4040  1556897
  10  49035  9404002  4040  1556897
  11  27538  9404000  4040  1556898
  12  27539  9404000  4040  1556899
  13  27540  9404000  4040  1556894
  14  27542  9404000  4040  1556900
  15  27543  9404000  4040  1556900
  16  27544  9404003  4040  1556901
  17  27546  9404004  4040  1556902
  18  99111  9404005  4040  1556903
  19  99112  9404006  4040  1556904
  20  99113  9404007  4040  1556905
  21  99116  9404008  4040  1556906
  22  99119  9404009  4040  1556907
  23  99122  94040010 4040  1556908
  24  99125  94040011 4040  1556909
  25  86007  94040012 4040  1556910
  26  86010  94040013 4040  1556911 
  </pre>
  
  <p>And when I try to perform a group by operation on the above dataframe, I get the "cannot reindex from a duplicate axis" error.</p>
  
  <pre><code>df.groupby(['GRP','CATG'],as_index=False)['PRD'].min()
  </code></pre>
  
  <p>I tried to find out the duplicate indices using:</p>
  
  <pre><code>df[df.index.duplicated()]
  </code></pre>
  
  <p>But didn't return any thing. 
  How can I go about resolving this issue?</p>