The following code optimises make_simplified_union in the common case
that there are exact duplicates in the union. In this regard, this is
similar to python#15104

To get this to work, I needed to use partial tuple fallbacks in a couple
places (these maybe had the potential to be latent crashes anyway?)
There were some interesting things going on with recursive type aliases
and type state assumptions

This is about a 25% speedup on the pydantic codebase and about a 2%
speedup on self check (measured with uncompiled mypy)

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