If you get this error response when shipping to Samos, it is because you have attempted to send more than one shipment with the same order reference ("Shipment Reference 1", in Shiptheory), which Samos does not allow.
To resolve this, click "Edit shipment" and scroll down to "Shipment Reference 1" and change it to a reference you have not used before for a Samos shipment and click Save.
