TST: Add test for duplicate `by` columns in `groupby.agg` using `pd.namedAgg` and  `asIndex=False`
- [x] closes #58446  
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit)


This PR adds a unit test to `pandas/tests/groupby/test_groupby.py` to verify the correct functionality of `groupby.agg` when passed in `pd.NamedAgg` objects with duplicate columns in the `by` argument, and with `asIndex=False`. Addresses GH #58446.
Thanks @jasonmokk 