TST: Test renaming a multiindex with duplicates (GH 38015)
- [x] closes #38015
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

This is my first real contribution, I appreciate the feedback!
Hello @andrewwoood! 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 2021-09-05 05:02:05 UTC Some pre-commit issues, otherwise lgtm. 

You could install pre-commit to run the hooks locally before pushing can you merge master and ping on green @jreback Sorry, is this what you were asking me to do? I'm not very competent with using git and contributing to open-source quite yet. I tried using the pre-commit framework but it doesn't seem to help my file pass the pre-commit check. The ci tells you whats wrong:

```
 pandas/tests/indexing/multiindex/test_multiindex.py:125:14: PDF019 found both 'pd.MultiIndex' and 'MultiIndex' in the same file
pandas/tests/indexing/multiindex/test_multiindex.py:126:14: PDF019 found both 'pd.DataFrame' and 'DataFrame' in the same file
pandas/tests/indexing/multiindex/test_multiindex.py:129:15: PDF019 found both 'pd.MultiIndex' and 'MultiIndex' in the same file
pandas/tests/indexing/multiindex/test_multiindex.py:130:20: PDF019 found both 'pd.DataFrame' and 'DataFrame' in the same file
``` Thanks @andrewwoood! Happy to have you address similar `good first issue`s that need tests.