BUG: MultiIndex dtypes incorrect if level names not unique
- [x] closes #45174
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Hello @johnzangwill! 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 2022-01-04 12:35:34 UTC > also pls add a test that exercises this

https://github.com/johnzangwill/pandas/blob/7777a8dae5c3485d588e19ccb098af57e0e79031/pandas/tests/indexes/multi/test_get_set.py#L70 thanks @johnzangwill 