Your buyer could not have bought and paid you for the item if it wasn't still listed for sale.
If you sold it beforehand, you should have removed it from sale.
This is the reason why eBay state, when you sell on their site, you must not advertise the item elsewhere, for exactly the situation you find yourself in, one item and 2 buyers.
You will need to first send a message to the buyer explaining what has happened, and that you will need to refund them.
Hopefully , they'll agree to the cancellation then you can cancel with their agreement, and no damage to your account. Click on item sold >> View order details top left>> More Actions on right>> Cancel item..... Buyer Agrees to cancel.
If they do not agree, you face damaging defects on your account, and possible negative feedback.
@margieeden