I guess you are talking about inbound deliveries, right?
When you are saying shipment, do you mean the transaction VT01N ?
How is the receipt posted? Via VL32N or MIGO, you see this in the header of the material document.
I can't really believe that it consequently comes to delivery of excessive material to the PO.
How can your vendor know that you have a problem with the status update? Why should he then deliver more than ordered?
If the physical movements match what you ordered then it will not come to any overdelivery, further has the PO fields for over- and underdelivery tolerances that actually control if you can receive more than ordered.