BUG: datetime ops with non-unique indexes is buggy
from [SO](http://stackoverflow.com/questions/25963622/column-operations-with-pandas-dataframe-objects-with-non-unique-indices/25964177?noredirect=1#comment40684868_25964177)

```
In [3]:       df = DataFrame({'A' : np.arange(5), 'B' : np.arange(1,6)},index=[2,2,3,3,4])

In [4]:   df.B-df.A
Out[4]: 
2    1
2    1
3    1
3    1
4    1
dtype: int64

In [5]:        df = DataFrame({'A' : date_range('20130101',periods=5), 'B' : date_range('20130101 09:00:00', periods=5)},index=[2,2,3,3,4])

In [6]:   df.B-df.A
Out[6]: 
2     0 days 09:00:00
2   -1 days +09:00:00
2     1 days 09:00:00
2     0 days 09:00:00
3     0 days 09:00:00
3   -1 days +09:00:00
3     1 days 09:00:00
3     0 days 09:00:00
4     0 days 09:00:00
dtype: timedelta64[ns]
```
