TST: Added iloc tests for casting from object to numeric dtypes for duplicate columns
- [x] closes #22035
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Hello @isaac-chung! 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 2021-04-25 22:57:11 UTC You could install pre-commit to automatically run the linting checks before committing I assume an extended test would not fall into the "whatsnew entry", so checking it off for completeness. Is there anything I can do about the failed checks? > I assume an extended test would not fall into the "whatsnew entry", so checking it off for completeness. Is there anything I can do about the failed checks?

no looks fine ping on green. Thanks @isaac-chung! The CI failure was unrelated