ENH:column-wise DataFrame.fillna and duplicated DataFrame.fillna with Series and Dict 
- [x] closes #4514
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

description:
Access "DataFrame" each column, fills up NA values using "Series.fillna" 
and what i found is if dataframe is duplicated, ".fillna" not guarantee filling NA. So i change them can fill NA.

Q. Why access column base not index base?
A. problem of Index base f".illna" is not preserve column's dtype. so i use column base.

Q. Why assign in column new values not use inplace=True ?
A. To avoid chained indexing. If column and index are both duplicated, chained indexing happens, i want to avoid this situation.

Hello @proost! 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 2020-08-10 17:20:16 UTC @proost can you fix merge conflict and try to fix CI failure? @proost is this still active? If yes can you merge master and fix the CI failure? Closing in favor of  #38352