Able to db-update the same package filename to multiple repos
We had a situation where a package with the same pkgver/pkgrel existed in two repos (community and community-testing).
While db-update checks if the package already exists in the pool, it's still possible for db-update to process two packages with identical filenames if they exist in different repos.
In other words, this will go through (and one repo will end up serving the wrong package):
staging/extra/foo-1-1-any.pkg.tar.zst
staging/testing/foo-1-1-any.pkg.tar.zst