PO's & Invoice's, create a default so that items are only ordered once

Where PO’s are concerned, it would be great if once you chose the vendor, you could only add items that pertain to that vendor. It would also be helpful if there was an alert that an item was on a PO and an invoice already so that we don’t have duplicate orders or billing a client for an item they have already paid for. It would also be nive if this feature was editable so you could make changes when needed. The current set up does not work for us at all.