Thank you for joining in this thread, @Tina_hgjones.

I can share additional insights in making sure you'll not get duplicate invoice numbers in QuickBooks.

Currently, the ability to receive a reminder or prompt if there are duplicates in your invoice number while creating it is not yet available. This option is only available with creating bills and checks. 

On the other hand, if you have not enable the Custom Transaction Numbers option, the system will automatically assigned invoice numbers. This is true whether you create invoices using the same template or different one's.

To verify your account's settings, kindly follow the steps provided by my colleague, @Anonymous.

Always feel free to visit us here in the Community if you have other questions about creating invoices using different templates in QuickBooks Online. I' m always here to help.