Two invoices( VF01) against a single delivery

Hi,

We facing problem when creating invoice in vf01. After doing delivery thru code VL10B, we create a proforma invoice through T-code VF01 . In our case, the user is able to make two proforma invoices against a single delivery.

Normally System will prompt error message "The document has already been fully invoiced"

But for below DO , 2 Invoices created , 1 Pro Forma Invoice & 1 Invoice , how did the system allowed that and what we shoud do to prevent it ?