Paypal Telling users The invoice has already been paid contact Merchant in PayPal Posted April 18, 2010 When I installed the IPN module I got the error and using this help text I fixed it. But it was an older version (I installed in 2007). This fix also worked for a similar problem I had with PayPal Standard. The coding is not exactly the same from IPN to standard in the respective files but the concept is there and I successfully made the adjustments to solve the issue for PayPal standard. FYI if someone using PayPal standard runs into this.