Orders passed through your SagePay 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.