[FIX] rolling to respect duplicate datetime indices on the right bound of centered windows
….

- [x] closes #43944
- [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 @rosagold! 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 2021-10-09 22:56:22 UTC lgtm. cc @mroeschke  Excellent find and PR! Thanks @rosagold 