Add test_masking_duplicate_columns
- [x] closes #31954
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I don't know what an whatsnew entry is...

This PR adds a test in order to solve an issue
Somehow I missed that, it's now fixed, thanks @dsaxton  Hello @GabrielSimonetto! 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 2020-10-18 18:14:20 UTC @GabrielSimonetto can you merge master and update to comments Closing in favor of #38354