Orders passed through your Opayo account must have unique IDs. If you experience this error when trying to checkout, it means the order ID that is being passed has been used before on your account. This can happen if you've switched to Kartris from another store system, so IDs are starting from a low level. It can also happen if you restored a older backup of your Kartris database for some reason.
The simple way to fix this is to set your database so that order numbers are allocated from a much higher based (above the range of any order numbers you might previously have run through the account). In SQL Management Studio, open up the tblKartrisOrders table in design view and view the properties of the O_ID field. There is a section for 'identity'. Within this, there is a seed value, which at default will be 1. Change this to a number in the tens of thousands, or hundreds of thousands, save the table changes. From now on, orders should be allocated much higher ID numbers, and these will be passed to SagePay as order IDs.
