This module checks that a supplier invoice/refund is not entered twice.
This is important because if you enter twice the same supplier invoice,
there is also a risk that you pay it twice !

This module adds a constraint on supplier invoice/refunds to check that
(commercial_partner_id, supplier_invoice_number) is unique, without
considering the case of the supplier invoice number. To activate that
feature:

- Go to Invoicing/Accounting \> Configuration \> Settings \> Vendor
  Bills
- Check the 'Check Unicity on Supplier Invoice Number' box.