The current interface, or perhaps the wording, can leave users wondering which question is the exact duplicate. So the current closure due to duplicate banner could be improved with a link to the question that is allegedly an exact duplicate. The possible duplicates box could remain up the top, but the text 'exact duplicate' in the closed banner should be a clickable link.
I'm assuming the current system closes a question as an exact duplicate if five users vote it is the duplicate of one other question. If they are voting it's a duplicate, but voting on different questions, then it should not be closed yet because there is still contention on why it is a duplicate. I feel this is necessary because there can be subtle differences in similar questions, and as far as I'm aware you can't cast a vote that a question isn't a duplicate of one of the possible duplicates.
Further, the possible duplicates shouldn't include a question which is already closed as a duplicate because this can make confusing cyclic linking such as in this question. All duplicates and their answers should point to some 'parent' duplicate.
Ironically, I think my own question here is a possible duplicate of this question, but the comments and answers on there don't really resolve things in my opinion. If we know which question it's an exact dupe of, what is the harm in having it clearly linked in the closure banner?