The merchant or seller is using the "invoice" variable in the item button code. If the merchant is not changing the invoice number dynamically each time the item button is used, the same number gets sent to PayPal each time. Because the system recognizes that the invoice has already been paid, should a second customer try to purchase the same item, he or she will get the following message: "This invoice has already been paid. For more information, please contact the merchant".
It's up to the merchant to turn this feature off in their PayPal account or generate a new invoice number each time. (They would need to select Profile, under My Selling Tools, Block Payments, select Update, look for Payment Receiving Preferences.)