BUG: bug in getitem with a duplicate index when using where (GH4879)

raising previously as this was not handled properly

```
In [1]: df = DataFrame(np.arange(12).reshape(3,4), columns=['A', 'B', 'C', 'D'],dtype='float64')

In [2]: df
Out[2]: 
   A  B   C   D
0  0  1   2   3
1  4  5   6   7
2  8  9  10  11

In [3]: df[df.C>6]
Out[3]: 
   A  B   C   D
2  8  9  10  11

In [4]: df = DataFrame(np.arange(12).reshape(3,4), columns=['A', 'A', 'C', 'D'],dtype='float64')

In [5]: df
Out[5]: 
   A  A   C   D
0  0  1   2   3
1  4  5   6   7
2  8  9  10  11

In [6]: df[df.C>6]
Out[6]: 
   A  A   C   D
2  8  9  10  11
```