Hi PLCPower,
I get that you need to view the supplier invoice number when paying your suppliers to avoid paying the same invoice twice. I recommend closing the Pay Bills window, then going to the Preferences window, clicking the Purchases tab, and selecting the Show Supplier Invoice Number in Supplier Payments option. Also, MYOB does have a system to warn of duplicate supplier invoice numbers. You may enable this by going to Setup > Preferences > Purchases and ticking the box indicating "Warm for Duplicate.". These actions will show the supplier invoice number instead of the purchase order and will prevent you from paying duplicate invoices. More details can be found in this helpful article about paying bills. Feel free to start a new post if you need further help.
Kind regards,
Shella