Allow same invoice numbers to be submitted by different providers

It would be good if different providers were able to create invoices with the same invoice number. It’s quite possible that two providers might use the same invoice number and since many providers have their own invoicing system if they are freelancers and work for other LSPs too (we have already come across this situation), it’s important that the invoice they raise in MultiTrans has the same invoice number as those recorded in their own system for reference, so really it would be better if the system only considers an invoice number to be duplicated if it is linked to the same provider.