Thanks for joining this thread, @KInch1. I'm here to provide information about invoice numbers.

Currently, the option to have a warning message for duplicate invoice numbers in QuickBooks Online (QBO) is unavailable. This feature is only possible with creating bills and checks. If in case you haven't enabled the Custom Transaction Numbers button, QBO will automatically assign numbers to the invoice. For more information on managing your invoices, feel free to check this article: Create invoices in QuickBooks Online. It also includes guides in accepting invoice payments. 

Also, you'll want to modify your invoice templates to suit your business needs and preferences. To do so, simply go to the Gear icon and select Custom Form Styles.

I'm always around here in the Community if you have any other invoice concerns. Feel free to post again here. Have a good one.