BUG: Fixed an issue with a duplicate index and duplicate selector with loc (GH4825)

[jreback]

closes #4825

In [1]: ser = Series([0.1, 0.2], index=[1, 2])

In [3]: ser
Out[3]: 
1    0.1
2    0.2
dtype: float64

In [4]: ser.loc[[3, 2, 3]]
Out[4]: 
3    NaN
2    0.2
3    NaN
dtype: float64

In [5]: ser.loc[[3, 3, 3]]
Out[5]: 
3   NaN
3   NaN
3   NaN
dtype: float64

In [6]: ser.loc[[2, 2, 3]]
Out[6]: 
2    0.2
2    0.2
3    NaN
dtype: float64

In [7]: ser.iloc[[1,1,0,0]]
Out[7]: 
2    0.2
2    0.2
1    0.1
1    0.1
dtype: float64