Hi, I read a different thread about this issue where another user had the same issue, but I couldn't find the follow up.
I am using Paypal sandbox with different Paypal and Joomla/HikaShop users buying different products and I am getting the error message in the subject title from Paypal every time.
I have set invoices Order Statuses to Created and see in orders that invoice numbers are being created. I have removed the block on duplicate orders from the PayPal side as per instructions, and I have edited the HikaShop order table to increment by 1000000. I don't think that has any effect since I am on the Starter edition of HikaShop though.
I have purchased HikaSerial and really need to be able to process an order so that I can make sure the serial number side of things is working properly.
Cheers
Paul