BUG/REG: multiindex nested tuples with duplicates
- [x] closes #30892
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

there is an underlying problem with MultiIndex.get_loc that needs to get worked out, but that's going to take a while.  This fixes the reported regression.