That is, I asked before to check the correctness of my proof of that statement, and the correctness has been confirmed. Could I now flag a question asking for the proof of it as "possible duplicate" of my question?
If your proof is the natural and generally accepted way to prove a certain statement, and it's hard to come up with alternative proofs, I'd say that's it's safe to close it as a duplicate.
If this is not the case, e.g. when your questions is asking to verify a proof that there are an infinite number of primes, it's certainly not appropriate to close it as a duplicate.