The existing ID + 1 isn’t changing between transactions, you’re incrementing the same ID multiple times.
There was a discussion on the forums previously. How to achieve Unique, Sequential Numbers Reliably? Where @keith goes into great detail about how to get a working solution.
For my solution, I created a 10 digital random alphanumeric string as the reference ID. Chances of the system generating the same 10 digital random alphanumeric code is quite low and due to transaction volume, I don’t see a collision happening anytime soon.