But it still should not be an issue, since according to what you quoted an acceptor will not reply to a second proposal that bears the same number ("If an acceptor receives... number n greater than...), which means only one of the two proposers can gather the required majority of promises to move to the next phase, and thus only one of the values can end up being accepted. After the crash you describe, there will be only one accepted value the Proposer-C will receive.
But it still should not be an issue, since according to what you quoted an acceptor will not reply to a second proposal that bears the same number ("If an acceptor receives... number n greater than...), which means only one of the two proposers can gather the required majority of promises to move to the next phase, and thus only one of the values can end up being accepted. After the crash you describe, there will be only one accepted value the Proposer-C will receive.