BUG: Groupby.Apply(Drop_Duplicates) seems buggy
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

a = pd.DataFrame({'a': [1,2,3,1,2,3], 'b': [2,1,2,1,2,1]})
print(a['a'].groupby(a['b']).apply(lambda x: x.drop_duplicates()))

a = pd.DataFrame({'a': [1,2,1,1,2,3], 'b': [2,1,2,1,2,1]})
print(a['a'].groupby(a['b']).apply(lambda x: x.drop_duplicates()))
```

Output:
```
0    1
1    2
2    3
3    1
4    2
5    3
Name: a, dtype: int64
b   
1  1    2
   3    1
   5    3
2  0    1
   4    2
Name: a, dtype: int64
```

#### Problem description

The two outputs have different schemas out when only changing the data. Expected that they would actually have the same levels of indicies. The first output should not "reset index" and completely lose the groupby context. 
