I created a sales order and then created a PO from the transaction for all the items. My co-worker then also created ANOTHER PO for the same items on the same sales order (they created through the sales order as well) There was nothing that popped up to say a PO was already created for these items/sales order. This means that this was double ordered. I went back to see if I could create yet another PO for the same items/sales order and IT LET ME!! 

Is there a way to stop duplicate  purchase orders from happening?

Previous programs I used would show a PO was already created/ordered or would have a pop up letting you know its already been ordered.