Fix bug in index.union with duplicates and not a subset of each other
- [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

The previous fix missed one case where other has dups which are unique in self

@jreback greenish, failure unrelated very nice @phofl 