They can indeed check the product file and shoulkd find that they have one more in stock than the file reads. They should also speak to the till person as their till number will also be on the reciept. Scanners at tills can be sensitive and read the barcode twice accidentally whilst the product is being scanned, but most GOOD sales staff would spot the error expacially if there were only a couplke of items in the transaction, Ring the store, give them the info of the reciept, i.e. product description as on receipt, till number, and time of transaction, then give them the barcode of the duplicated item, let them know you want a CREDIT onto your card (if you paid cash you want the cash) and tell them when you are going in, take the reciept and goods with you so that they can process the credit,(it is not technically a refund it is an overpayment)