When a question gets closed as an exact duplicate, I often see that the original, which the question gets closed as a duplicate of, is itself closed.
This is not very nice behaviour.
My proposal is for the system to check, when a question gets closed, if the original is itself closed, and if so, don't show a link to that original, but to the original's original.
Or in other words, if question B is closed as a duplicate of A
(and B has a link "Possible duplicate: A") then when question C gets closed by people as a duplicate of B, let the system show a link to question A instead of B.
So that new readers of question C won't end up in another closed question when they click the "Possible duplicate" link.