REF: use concat_compat in union_with_duplicates
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

wow

any perf implications? > any perf implications?

we're going to take a small hit bc isinstance(obj, ABCFoo) is slower than isinstance(obj, Foo), and the runtime import of ensure_wrapped_if_datetimelike (i guess could move that to dtypes.cast).  the union cases we'll take a hit in non-EA cases bc concat_compat is slower than np.concatenate, but for EA  cases we ought to do better > > any perf implications?
> 
> we're going to take a small hit bc isinstance(obj, ABCFoo) is slower than isinstance(obj, Foo), and the runtime import of ensure_wrapped_if_datetimelike (i guess could move that to dtypes.cast). the union cases we'll take a hit in non-EA cases bc concat_compat is slower than np.concatenate, but for EA cases we ought to do better

sounds good about moving that, but this looks fine.