Sage 100 Contractor should have an option to make the duplicate invoice warning an error message which prevents a data entry person from entering a duplicate invoice.

Many Data entry people get used to clicking Yes to warning messages without reading them. Warning messages are not sufficient to stop duplicate entries.

There needs to be a setting that stops them from entering duplicates if the vendor record doesn't allow them.

The "Allow duplicate invoices" check box on the vendor record is pointless in a duplicate can still be created when this box is not checked.