BUG: groupby with levels on duplicated multi-index
```
x = pd.DataFrame({'x':[1,1,3,3],'y':[3,3,5,5]},index=[11,11,12,12])
y = x.stack()
print(y)
print(y.groupby(level=[0,1]).sum())
```

Ouput

```
11  x    1
    y    3
    x    1
    y    3
12  x    3
    y    5
    x    3
    y    5
dtype: int64
11  x    1
    y    3
    x    1
    y    3
12  x    3
    y    5
    x    3
    y    5
dtype: int64
```

The stack and group by sum are just the same.

Expected output:

```
11  x    2
11  y    6
12  x    6
12  y    10
```
