The globalblocks table has an index named gb_address which is used to lookup by target and also enforce uniqueness. However, to allow global autoblocks (T368949) we need to be able to have both an autoblock and non-autoblock target the same IP address.
This brings feature parity with core, which allows an IP to be both autoblocked and blocked directly. This also is necessary because it would be easy to determine the target of an autoblock if it gets removed for clashing with a global block on an IP address.
Therefore, the gb_address index needs to be updated to allow rows with the same target but where one is an autoblock and the other is not.