Test added to check if MultiIndex is unique
- [X] closes #44786
-  1 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
- New test added to check if MultiIndex is unique

closing in favor of #45822 