Hi there, @jma2019.

Thanks for following the thread. 

As of now, all of our users should receive a prompt that you're using a duplicate invoice number. The prompt will look like the screenshot I've included below. 

If for any reason you're not being prompted of the duplicate invoice number, I recommend contacting our Support Team. This way,  an agent can review your account securely and investigate this matter further. 

I hope this update finds you well. Please don't hesitate to let me know if you have any questions or concerns. I'm always around to lend a hand. Take care!