Purchase order number xxxxx has already been used in business transaction xxxxx

Hi Friends,

We are getting the below error in Sales order in CRM, while order is created in ECC and replicated to CRM. It's happening as the same purchase order number is used as reference in two different sales orders, but sales order was completed without errors in ECC. Whether we should not use the same purchase order number in two sales orders? Please help why its happening and how to resolve it.

"Purchase order number xxxxx has already been used in business transaction xxxxx"

Thanks and Regards,

VR