TST: Add test for `GroupBy.describe()` with duplicate columns
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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While working on #35314 I made a change that broke `GroupBy.apply` if you called it on a DataFrame with duplicate columns; however, this break as not caught by any test. Was just caught on inpsection: https://github.com/pandas-dev/pandas/pull/35314#discussion_r460000612

This issue is just to add a test for this case to ensure it works and doesn't break again in the future. 