Duplicate index with FloatIndex giving 'ValueError: Length mismatch'
Not certain if this is a bug or defined behaviour (but then the error message is not clear in any case).

In 0.13.1:

```
In [28]: df = pd.DataFrame(np.random.randn(9).reshape(3,3), index=[0.1,0.2,0.2],
 columns=['a','b','c'])
In [29]: df
Out[29]:
            a         b         c
0.1  1.711117  1.218853 -1.322363
0.2  0.956266  0.230374 -1.005935
0.2 -0.137729 -0.993931 -0.902793

In [30]: df.ix[0.2,'a']
Out[30]: array([ 0.95626607, -0.13772877])

In [31]: df.ix[0.2]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
...
ValueError: Length mismatch: Expected axis has 0 elements, new values have 2 ele
ments
```

In master, both (`df.loc[0.2]` and `df.loc[0.2, 'a']`) give this error message. Wile for integer index, this works.
