BUG: Remove error raise to allow same input function on the same column in named aggregation
- [x] closes #28426 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


https://github.com/pandas-dev/pandas/pull/27921#issuecomment-521794197

this was kind of mentioned in the PR27921, and the case used in the issue is basically the same as having `min` or `max`. Previously it was not raising error is because I deliberately skipped this case since i thought this removal should be treated in different PR. Feel free to review and comment your ideas.
