You mean the invoice number of your supplier? You can't even enter your own incoming invoice number (i.e. your document number) in Projo - this is automatically assigned and I currently don't know any way to change it in Projo. With the supplier's invoice number: Do you mean a check, per supplier, that no numbers may occur twice (message then, for example, “An invoice from supplierXY with the invoice number XYZ has already been entered in projo”). This is because the number could easily be duplicated by chance between different suppliers.