Sort()ing then selecting columns in a function apply()d to a grouped DataFrame
This code:

``` python
import pandas as pd
from numpy.random import randn


df = pd.DataFrame(randn(8, 4), columns=['A', 'B', 'C', 'D'])

def function(group):
    group.sort(columns = 'C', inplace=True)
    group = group[['A', 'B', 'C', 'D']]
    return group    

df2 = df.groupby(['A']).apply(function)
```

produces this error:

``` python
ValueError: cannot reindex from a duplicate axis
```

It's the combination of the sort and the column selecting inside of a grouped apply that causes the problem. I'm happy to give more detail on why I want to do this, but this is the simplest most striped down code that produces the error.
