BUG: groupby _cython_agg_blocks implicitly assumes unique columns
xref #31616, the two test cases that adds both have unique columns.  Editing test_agg_split_object_part_datetime to make columns non-unique breaks it:

```
df = pd.DataFrame(
            {
                "A": pd.date_range("2000", periods=4),
                "B": ["a", "b", "c", "d"],
                "C": [1, 2, 3, 4],
                "D": ["b", "c", "d", "e"],
                "E": pd.date_range("2000", periods=4),
                "F": [1, 2, 3, 4],
            }
).astype(object)
df.columns = ["A", "B", "B", "D", "E", "F"]

>>> result = df.groupby([0, 0, 0, 0]).min()
pandas.core.indexes.base.InvalidIndexError: Reindexing only valid with uniquely valued Index objects
```

cc @WillAyd @TomAugspurger 