This article addresses the error message "Payment has already been made for this InvoiceID" that can occur during a PayPal transaction. This error message is closely related to the use of reference numbers in PayPal transactions.
PayPal expects a unique reference number for each transaction. This reference number is transmitted to Computop via the "RefNr" parameter. The "RefNr" is transferred to the "InvoiceID" field in PayPal. If a payment is attempted with an already used "InvoiceID", PayPal issues the error message "Payment has already been made for this InvoiceID". This means that a payment has already been made with this specific "InvoiceID".
To avoid this error message, ensure that each transaction sent to PayPal has a unique "RefNr". This can be achieved by implementing a system that automatically generates unique "RefNr" and uses them for each transaction. By using unique "RefNr", it ensures that each transaction sent to PayPal is recognized as a new and unique transaction, thereby avoiding the error message "Payment has already been made for this InvoiceID".