REGR: MultiIndex Indexing
- [ ] closes #35351
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I'll add tests if nothing breaks. I'm not keen on special casing the base Index method, but overriding _reindex_non_unique maybe overkill. maybe better to revert part of #33691 test failures unrelated, see #34850 so marking as ready for review. whatsnew needed for regression in rc? Thanks, LGTM.

> whatsnew needed for regression in rc?

I don't think so. mpl issue breaking the docs? rebased and green. since this is for 1.1, I suppose this could be merged even without a firm decision on the branch strategy. Yep, thanks.