Customer Cross Reference - Address Level - Duplicates desired

How do I set system to allow duplicates in customer item cross reference at address level?  Currently we load at address level. I need to be able to use same customer item on multiple items as the customer revises the part and we manage this data in the customer cross reference description.  We have programmed the data to print on Certificates of Compliance that we generate at time of shipment (we make medical packaging).  Our customers have the requirment that we print their part number on our documents, but do not want to change their part number when they revise the