BUG: Allow Series with same name with crosstab (#13279)
 - [x] closes #13279
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Before, passing two `Series` with the same name in `crosstab` raised a `KeyError` because of a block in `pivot_table` that tried to unstack a `MultiIndex` with duplicate names. 