There can be only one (tmpfs mount)

Well, that’s not exactly correct: you can mount more than one tmpfs, and you can mount multiples at the same spot, but I can’t think of a reason to do so.  In fact, it could happen by accident, but there’s a fix for that in DragonFly, thanks to Aaron LI.  Not a major problem, but mentioning it in case you saw it and were confused.