Groupby filter doesn't work with repeated index
Example:

```
In [1]: data = pandas.DataFrame(
    {'pid' : [1,1,1,2,2,3,3,3],
     'tag' : [23,45,62,24,45,34,25,62],
     }, index=[0] * 8)

In [2]: g = data.groupby('tag')

In [3]: g.filter(lambda x: len(x) > 1)
Exception: Reindexing only valid with uniquely valued Index objects
```

cc #3680

_(I had thought there was also a similar behaviour with transform, but that seems ok.)_
