If a question A is closed as unclear, it means that it is unknown what the asker meant. (Otherwise the question would have been edited rather than closed.)
If some question B is unclear (also if it's written similarly to A), it can be closed as unclear. If question B is clear, it is unknown whether B is a duplicate of A because it is unknown what question A means (see above).
So it should be impossible to mark a question B as a duplicate of an unclear-closed question A. Why is it possible?
Edit: If B is very similar to A, then closing it as unclear or as duplicate both is appropriate. The latter is easier/faster as explained in this comment. And this is a problem. Because if A and B are somewhat dissimilar, this creates an unnecessary dilemma of choosing either speed (closing as duplicate) or clean logic (closing as unclear).
Edit regarding possible duplicate of this question: I'm asking why an action that depending on the situation is either illogical or unnecessary is technically possible. The question you marked as duplicate of mine asks about formulations in the duplicate policy regarding "broad" and "narrow" questions. The other question marked as duplicate is about a bug in the vote calculation. These are all different questions!