Why does the OrderID needs to be unique?
Some of our customers choose not to store ICEPAY's PaymentID in their system. When they receive a Postbank Notification they rely on the OrderID to correctly identify the order in question. To prevent any miscommunication ICEPAY decided to require a unique value for OrderID per MerchantID.
If you are using OrderID's that have been used in the past you should increment your internal OrderID to above the values previously used.