iSupplier Portal - It allow to create 2 invoice for same po line and same amount

Dear All,

iSupplier Portal - It allow to create 2 invoice for same po line and same amount.

For e.g  PO - P123  PO Line - 1   PO qty - 10

Invoice - INV1  Invoice qty - 10  Po Line - 1

Invoice - INV2   Invoice qty - 10 Po line - 1

Is their any way to stop it in oracle ? 

Regards

Sandeep Yadav