Hi Sharrynne and KATESOLECT
Thanks for your post. It's great to have you here in the Community forum.
Currently, MYOB Business/Essentials doesn't have the ability to warn for duplicate invoice numbers when entering the invoice with the same reference number.
However, I would suggest posting these functions that you would like to see implemented into the system on our MYOB Business/Essentials Idea Exchange board.
Please don't hesitate to post again if you need help in the future.
If my response has answered your enquiry please click "Accept as Solution" to assist other users find this information.
Cheers,
Leneth