Can you see anything related to this in the mail log?
BTW, one person receives an autoresponder message only once. For example, user1 has set up an autoresponder. Shortly afterwards, user2 sends an email to user1, and user2 receives the autoresponder message. Then user2 sends another message to user1, but this time user2 does not get the autoresponder message again because he has already received it once.