TST/BUG: duplicate indexing ops with a Series using where and inplace add buggy (GH4550/GH4548)

closes #4550,#4548

```
In [1]: s = pd.concat([Series(list(range(3))),Series(list(range(3)))])

In [2]: s
Out[2]: 
0    0
1    1
2    2
0    0
1    1
2    2
dtype: int64

In [3]: s.where(s<2)
Out[3]: 
0     0
1     1
2   NaN
0     0
1     1
2   NaN
dtype: float64

In [4]: s[s<1] = 5

In [5]: s
Out[5]: 
0    5
1    1
2    2
0    5
1    1
2    2
dtype: int64

In [6]: s[s<2] += 10

In [7]: s
Out[7]: 
0     5
1    11
2     2
0     5
1    11
2     2
dtype: int64
```