Refunds have to be made through a Case of some sort. Just sending money back is not recognised as a refund, so if buyer has opened a Not Received Case ebay still expect you to refund,and if buyer escalates they will refund again.
So not only have you got this problem,you have a defect for refunding incorrectly and will still have to pay the selling fee.
You need to get the buyer to close whatever case he has opened by going to the Resolution Centre. That will at least get ebay off your back.
