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. 
@smithto1 tests are failing

```
=========================== short test summary info ============================
FAILED pandas/tests/groupby/test_function.py::test_describe_with_duplicate_output_column_names[True]
FAILED pandas/tests/groupby/test_function.py::test_describe_with_duplicate_output_column_names[False]
=== 2 failed, 70016 passed, 4046 skipped, 1022 xfailed in 547.03s (0:09:07) ====
``` Hello @smithto1! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-08-03 20:58:22 UTC thanks @smithto1 very nice