We are currently using Enterprise Contractors Edition 24.0. We've been using it for 15 years.

It has always been that if 2 people are entering invoices for customers at the same time and they ended up on the same invoice number (lets say invoice 12345), as soon as the 2nd person went to save, print, or email the invoice a message would pop up that said something to the effect of "another user has saved an invoice with number 12345, your invoice will be saved as 12346" and it would automatically change the invoice number to one that hadn't been used. 

We have not changed anything, but now it is not changing the invoice number to an unassigned one and allows a duplicate invoice number to be used. This can be an absolute disaster in our system.

How do I stop allowing it to let duplicate invoice numbers happen?