TST: adding test for .describe() with duplicate columns
- [x] closes #35423 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

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 was not caught by any test. Was just caught on inpsection: https://github.com/pandas-dev/pandas/pull/35314#discussion_r460000612

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