BUG: duplicate indexing on non-integer index with positional indexers failing in py3
#### Code Sample, a copy-pastable example if possible

> > > df = pd.DataFrame({"a" : [0,1,2], "b" : [1,2,3]})
> > > df[["a", "a"]].apply(lambda x: x[0] + x[1], axis = 1)
> > > #### Expected Output
> > > 
> > > 0    0
> > > 1    2
> > > 2    4
> > > dtype: int64
> > > #### output of `pd.show_versions()`
> > > 
> > > Traceback (most recent call last):
> > >   File "C:\Users\Alexander\Anaconda3\lib\site-packages\pandas\indexes\base.py", line 1980, in get_value
> > >     tz=getattr(series.dtype, 'tz', None))
> > >   File "pandas\index.pyx", line 103, in pandas.index.IndexEngine.get_value (pandas\index.c:3332)
> > >   File "pandas\index.pyx", line 111, in pandas.index.IndexEngine.get_value (pandas\index.c:3035)
> > >   File "pandas\index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas\index.c:3955)
> > >   File "pandas\index.pyx", line 169, in pandas.index.IndexEngine._get_loc_duplicates (pandas\index.c:4236)
> > > TypeError: unorderable types: str() > int()

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Alexander\Anaconda3\lib\site-packages\pandas\core\frame.py", line 4061, in apply
    return self._apply_standard(f, axis, reduce=reduce)
  File "C:\Users\Alexander\Anaconda3\lib\site-packages\pandas\core\frame.py", line 4157, in _apply_standard
    results[i] = func(v)
  File "<stdin>", line 1, in <lambda>
  File "C:\Users\Alexander\Anaconda3\lib\site-packages\pandas\core\series.py", line 583, in __getitem__
    result = self.index.get_value(self, key)
  File "C:\Users\Alexander\Anaconda3\lib\site-packages\pandas\indexes\base.py", line 2000, in get_value
    raise IndexError(key)
IndexError: (0, 'occurred at index 0')

This is the error I get when running on Python 3.5.1/Pandas 0.18.1 on Windows. I get the expected output on Linux running Python 2.7.11/Pandas 0.18.1.
