[BUG] Bad results for Groupby.get_group when the index has repeated values

[rjzamora]

Describe the bug
Using Groupby.get_group produces bad results when the selected group has repeated index values.

Steps/Code to reproduce bug

import cudf

df = cudf.DataFrame(
    {"a": range(10), "b": [0] * 10},
    index=[0] + list(range(9)),
)
pdf = df.to_pandas()
pdf.groupby("b").get_group(0)

Pandas returns expected behavior:

   a  b
0  0  0
0  1  0
1  2  0
2  3  0
3  4  0
4  5  0
5  6  0
6  7  0
7  8  0
8  9  0

df.groupby("b").get_group(0)

cuDF duplicates all rows with a 0 index:

   a  b
0  0  0
0  1  0
0  0  0
0  1  0
1  2  0
2  3  0
3  4  0
4  5  0
5  6  0
6  7  0
7  8  0
8  9  0

Additional context
The bug seems to be in this line. That is, I'm pretty sure this line needs to change to something like:

return obj.loc[self.groups[name].drop_duplicates()]