Dont close questions as duplicate if the original is closed.
I've seen a lot of questions closed as duplicates of already closed questions, in most cases as duplicates themselves. Unless it was closed for migration, it seems pointless to direct people instead to a question that probably won't help.
If the other question is a duplicate, the system could offer to use the question at the end of the line. If it's for another reason, it could suggest voting to close the target for the same reason.