We recently had work completed on two identical oscommerce sites. This included adding Paypal and replacing the standard Order Number with a new Order Number. Since the work was completed we have been receiving the error - Invoice already paid when attempting to pay via Paypal. The new order number is also not being transferred to paypal.
1. Fix Error - Invoice Already Paid on two identical sites.
2. Update code to send new order number to Paypal for both sites.
As we are using two identical sites, I'm assuming the old order number is causing duplicate order ID's in Paypal resulting in the error.