But warn about them. If somebody really ends up later wanting to
explicitly add a note that something has the same parent twice (who
knows, there are strange people around), we can add a flag to say that
it's expected and ok.
This was brought on by a commit in the kernel tree, where a repeated
merge caused a duplicate parent.
Parent duplicates aren't "wrong" per se, they're just in practice not
something you are ever interested in.