Add a check for duplicate union members

[Akuli]

Discussed in microsoft/pyright#2753.

The code for detecting Union[str, str] is a bit fragile, as it assumes you do from typing import Union. I can remove it if that is better. Typeshed uses | syntax anyway.

[JelleZijlstra]

Thanks! I think the typing.Union check is fine, but it would be good if the error message said what the duplicate member is (e.g. 'Duplicate enum member "str"')—it may not be obvious if you have a very complicated Union.

Out of curiosity, did this find any other instances in typeshed?

[Akuli]

I improved the error message. It found these:

stdlib/difflib.pyi:8:46: Y016 Duplicate union member 'Callable[[str], bool]'
stdlib/argparse.pyi:295:37: Y016 Duplicate union member 'Callable[[str], _T]'
stdlib/argparse.pyi:314:41: Y016 Duplicate union member 'Callable[[str], _T]'

The first two have Text and str in Python 2 stubs, and str and str in Python 3 stubs. The last one is in a Python 3 only class that also had Text and str for some reason, but python/typeshed#5464 changed both to str.

[srittau]

LGTM