DataFrame.unstack() issues DeprecationWarning with MultiIndex
``` python
>>> import warnings
>>> warnings.simplefilter('error')

>>> import pandas as pd
>>> pd.__version__
u'0.17.0rc1'

>>> df = pd.DataFrame({'A': list('aabb'), 'B': [0, 1, 0, 1], 'C': range(4)})
>>> gf = df.groupby(['A', 'B']).sum()
>>> gf
     C
A B   
a 0  0
  1  1
b 0  2
  1  3

>>> gf.unstack()
Traceback (most recent call last):
..., line 4274, in _reference_duplicate_name
    return np.sum(name == np.asarray(self.names)) > 1
DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
```
