PayPal returns a duplicate transaction error even though it is not a duplicate. This usually happens when the same PayPal account has been used on more than one AShop installation.
1. There is a Block accidental paymentssetting at PayPal.com under Profile > Payment Receiving Preferences. Change it to allow multiple payments per invoice ID.
2.
Change the starting orderid number to a higher number such as 1000 or 10000. This can be done from a command line or with the aid of a database management tool such as phpMyAdmin.