Oh dear you should never refund money before it's released from Airbnb.
It sounds like the guest will have been refunded twice. Once through Airbnb and once from you . (If I have understood you correctly)
all you can do is contact the guest explain the situation and hope they will be honest enough to pay you back the over payment . @Kevin3261
Or are you saying the guest just had the refund from you and that no payment came through from Airbnb? Was it you or the guest that cancelled the booking?