Double producing. Whether it is intentional or a problem of misconfigured backup node, using the same or different witness signatures, now it can only happen once per schedule.

One block producer double voting isn't going to make anything irreversible, because a 3/4ths majority is needed. The problem case here would be if a LOT of witnesses were double producing. This is part of the reason we don't just use something like a simple majority for irreversibility.

There is one scenario where that could happen: if a lot of witnesses configured a backup producer to "fail-over" produce if the backup loses contact with their primary producer node. Of course, such a node configuration has always been a bad idea and could always result in irreversible forks along the split line if it managed to split backup nodes on one side and primary nodes on the other> But the proper setup for a backup node is to have it ready to run, but require manual intervention to enable block production, so that a network split doesn't result in two irreversible forks.