BUG: _nsorted incorrect with duplicated values in index (#13412)
 - [X] closes #13412
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [X] whatsnew entry

`DataFrame.nlargest` and `DataFrame.nsmallest` does not behave correctly when there's duplicate values in the index. 

Any feedback is appreciated!