double entry of service entry sheets in shipment cost document

Dear friends,

I have a probem in the shipment costs.

Service entry sheet is created twice for the same shipment cost document.

First time the service entry sheet is created for one PO and invoice and payment to vendor is completed for the same.

Second time, for the same shipment cost,service entry sheet is created and updated in a different PO. Now the problem is since the payment to vendor is completed for the first PO. The newly created service sheets remain open.

How to avoid the open service entry sheets from being paid to vendor.

waiting for your valuable reply.

regards