Obviously you know it's your fault but if the buyer's insisting on two when you've explained you've made a mistake and there's only one available, I think you may have a problem buyer

I'd cancel both transactions and block the buyer. Others may think differently

Cancel a transaction

https://www.ebay.com.au/help/selling/getting-paid/sellers-can-cancel-order?id=4136&srsltid=AfmBOorXO...

Block a Buyer

https://www.ebay.com.au/help/selling/resolving-buyer-issues/blocking-buyer?id=4082&srsltid=AfmBOoprg...