Bug in melt raising Error with dup columns as value vars
- [x] closes #41951
- [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

@simonjayhawkins I think this changed in May last year, so probably not woth backporting?