1. In Configuration, click Business Rules
2. Highlight Invoice.
3. Under Duplicate invoice criteria, Add Invoice number to Field Name column
4. Add 20 (or some other large number) to Length column. *
5. Remove additional rows that exist by right-clicking and selecting Delete row. *
6. Check box for Automatically check for duplicate invoices
7. Choose an option from the drop down box for Warn user or Do not allow duplicates

*Note: Adding 20 to Length is telling The Financial Edge to match the Field Name up to 20 characters. If an invoice number is very long, and Length is set to a smaller number, duplicates may still occur.
          If additional rows are added, the filter becomes more narrow and duplicates may still occur