You need to get on the phone to ebay and keep asking for a superviser until you get somebody who understands what is going on.

If no joy there call Paypal, their suoervisers are uaually very good.

If that doesn't work either contact the FSO, you can do it on line.

In future if a buyer opens a case only refund through the dispute.

I think you are being a bit over the top accusing the buyer of fraud. Post to Mexico is a problem so I would not post there unless I could use fully tracked post. Chances are it has been pinched before it gor anywhere near the buyer. Have you asked the buyer to return one of the payments? If I received 2 refunds I doubt if I would notice among all the other transactions which go through my Paypal.