BUG: Avoid duplicating entire exploded column when joining back with origi…
…nal frame

- [x] closes #28005
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @MarcoGorelli! 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 2019-09-15 23:06:12 UTC Thanks for your help Marc, really appreciate it! @MarcoGorelli looks like a git merge issue. Can you try fetching and merging master, and then repushing? Let us know if you get stuck. @TomAugspurger sorry to chase you up on this - I've resolved the merge conflict, is there anything else you'd like changed? Thanks!