BUG: Fix DataFrame.astype(ExtensionDtype) with duplicate column names
- [X] xref https://github.com/pandas-dev/pandas/issues/24704#issuecomment-453266869 (this fixes item 1)
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
