groupby().apply() executes the first item twice
Pandas 0.14.1
How to reproduce. 
In the example below, the first item df['a'] = 1 is executed twice.
$ import pandas as pd
$ f = pd.DataFrame({'a':[1,2,3,1,1,2], 'b':[1,2,3,4,5,6]})
$ def fx(df):
      print df
      return df['b'].sum()

$ f.groupby('a').apply(fx)
   a  b
0  1  1
3  1  4
4  1  5
   a  b
0  1  1
3  1  4
4  1  5
   a  b
1  2  2
5  2  6
   a  b
2  3  3
