In Paxos, can an Acceptor accept a different value after it has already accepted one?

In Multi-Paxos algorithm, consider this message flow from the viewpoint of an acceptor:

receive: Prepare(N)

reply: Promise(N, null)

receive: Accept!(N, V1)

reply: Accepted(N, V1)

receive: Accept!(N+1, V2)

reply: ?

What should the acceptor's reaction be in this case, according to the protocol? Should it reply with Accepted(N+1, V2), or should it ignore the second Accept!?

I believe this case may happen in Multi-Paxos when a second proposer comes online and believes he is (and always was) leader, therefore he sends his Accept without Preparing. Or if his Prepare simply did not reach the acceptor. If this case may not happen, can you explain why?