BUG: df.loc setitem-with-expansion with duplicate index
- [ ] closes #xxxx
- [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

also CategoricalIndex.insert
cc @phofl I think we should deprecate indexing expansion, pretty sure we have an issue for this. updted per comments + greenish