BUG: DataFrame.__setitem__ raising ValueError when setting multiple values to dup columns
- [x] closes #15695
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Setting with a scalar key which is duplicated in columns should be the same as using a listlike key