DEPR: Allowing grouping by an index label when there are duplicates

[rhshadrach]

From #49373 (comment)

df = pd.DataFrame(
    {
        "a": [1, 1, 2],
        "b": [3, 4, 5],
    },
    index=pd.MultiIndex.from_tuples([(1, 1), (1, 2), (1, 3)], names=["f", "f"]),
)
gb = df.groupby("f")
result = gb.sum()
print(result)
#    a   b
# f       
# 1  4  12

There are two levels with the label "f" here; I think what the user wants in such a case is ambiguous. As such, we should raise instead of using the first level with the given label. In the similar case of grouping by columns, we currently raise.