How can we stop ERS from creating invoices for receipt for which invoices have already been created

Hi Experts,

I am facing problem while using ERS i.e. if any user creates manually creates an invoice and matches it to a PO which is marked for self billing, the system gives a warning that "This PO is marked for self billing and creating this invoice will result in an over payment to your supplier. However it does not stop the user from creating invoice if user overrides the warning and later on when "Pay on receipt auto invoice" Program runs it once again creates the invoice for the same PO line and receipt number. How can we stop the system