Invoice number uniqueness in iSupplier portal

 How do we retain invoice number uniqueness at the invoice level versus supplier level?

We have some large suppliers who are using multiple systems to enter invoices in iSupplier portal.

As uniqueness is maintained at vendor(Supplier) level, how can we offer capabilities to vendor to enter same invoice number? - Is it possible to prepopulate invoice number field in iSupplier portal that will be appended by the vendor with their own number at the end.

For e.g. prepopulate with Alphanumberic string of characters like ABCD0001- and then vendor can append last 5 /7 characters of their own invoice number at the end as ABCD0001-INV230