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In Paxos, can an Acceptor take on a different meaning after it has already accepted it?

In the Multi-Paxos algorithm, consider this message flow in terms of an acceptor:

receive: Prepare (N)

reply: Promise (N, null)

receive: Accept! (N, V1)

reply: Accepted (N, V1)

receive: Accept! (N + 1, V2)

answer:?

What should be the reaction of the acceptor in this case, according to the protocol? Should he answer Accepted (N + 1, V2) or should he ignore the second Accept !?

I believe that this case can happen in Multi-Paxos, when the second interlocutor goes online and believes that he (and always has been) a leader, so he sends his Acceptance without preparation. Or, if its preparation simply did not reach the acceptor. If this case cannot happen, can you explain why?

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My real understanding is that I should not accept a value in N + 1 (i.e. do not reply or send NACK at all), thereby forcing the leader, perhaps, to start another round using Prepare (if most have not reached consensus so far ) After receiving Prepare (N + 2), I will answer Promise (N + 2, V1) and continue as usual.

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