Phillip Wood [off-list ref] writes:

quoted

One.  Have you considered the case where the remote-tracking refs
are overlapping, e.g., where "origin" and "upstream" point at
different URLs but they both store in "refs/remotes/upstream/*"?
Perhaps their URLs may textually be different but are pointing
logically at the same place (e.g., one ssh:// the other https:// for
example).

What should happen?  What does happen after you apply this patch?

It would be worth looking at what "git checkout --track" does in that 
case and seeing if we can share the code.

It always is a good idea to think how we can share code for
different purposes to solve a new problem, but in this particular
one, I am not sure if "git checkout -t -b topic upstream/main"
codepath has much to offer to solve what the new "before the
checkout, update from the remote" feature wants to do.  To the
former, it does not matter how refs/remotes/upstream/* are updated
and by fetching which remote at all.  The only thing it cares about
is to leave the record that this new "topic" branch works with
refs/remotes/upstrea/main.  But the latter needs to be able to
compute which remote it should fetch from.  It is a problem that
existing code had no need to solve.