It shouldn’t get duplicated, the latest update to B1357 was supposed to fix this
As for autoquitting the server, you can’t. There are many advantages to running the server as a separate process, as laid out in the thread linked below. And I can’t think of any disadvantages to have it running when you don’t play Roon - as its inactive, the operating system should swap it to disk in the event of memory pressure and it shouldn’t be using CPU in any noticeable way.
I guess if you really want to you can find a way to autokill it