When you say Duplicate Purchase, did the buyer purchase the same item twice from you? Or did they make two purchases and have now decided they don't want to pay for one of them?

You have 2 options.

Add the buyer to your blocked list. Let the Unpaid case continue and in another 2 days you'll get your FVFs refunded and the buyer will receive a strike. No feedback can be left.

Or send a Mutual Cancellation using the reason "buyer changed mind" and you shouldn't receive a Defect. IF the buyer accepts it you'll get your FVFs refunded (or if they ignore the request you can get your FVFs refunded after 7 days). If they decline the cancellation request you'll need to ring eBay and ask for your FVFs to be refunded based on the buyer's emails. The buyer can still leave feedback and DSRs which may leave you with a Defect.