BUG: resample.apply with non-unique columns
- [ ] 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

i guess a bug, an you add a whatsnew. lgtm otherwise, ping on green-ish. ping sweet.  once the other groupby stuff goes in, i think this was the last bugfix before we're ready for some nice de-duplication