BUG: can't concatenate DataFrame with Series with duplicate keys
- [x] closes #33654
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

lgtm. let's merge on green. Thanks - sure, it's green now Thanks @MarcoGorelli 