control and a test of control

are the tests of controls given for the stated control valid?

Control: When an invoice is sent to the customer, the system marks the system generated GDN as “INVOICED” to prevent a customer being invoiced twice.

Test of control:

1. enter a dummy GDN and then raise an invoice to check if the GDN has been marked as invoiced.

2. select a GDN for which invoice has been generated and try to generate the invoice again to see if the system allows for the second invoice to be generated.