Praise be to Allah.
He should respond to the first one and to the second, because of the general meaning of the words of the Prophet (peace and blessings of Allaah be upon him): “When you hear the muezzin, then say what the muezzin says.”
But if he has prayed then he hears a muezzin after he has prayed, then the apparent meaning of the hadeeth is that he should repeat after him, because of its general meaning.
Some of the scholars said that he may not repeat after him because he is not being called in this adhaan, so he does not have to repeat after him, and it is not possible for another call to prayer to be given after the prayer has been offered, so the hadeeth is to be interpreted according to what was usual at the time of the Prophet (peace and blessings of Allaah be upon him), when the adhaan was not repeated. But if someone wants to follow the general meaning of the hadeeth and say that it is dhikr, and as the hadeeth is general in meaning, then there is no reason why one should not remember Allaah, and that is fine. End quote.