Turn it on for each sub-calendar that needs it
Double-booking prevention is a setting on a sub-calendar, not on the whole calendar. A meeting room, a van, a treatment room or a court turns it on. A team’s calendar usually leaves it off, because two meetings in one team at the same time is a busy afternoon, not a clash.
With it on, a booking that would overlap another on that sub-calendar is refused when someone tries to save it. The person saving it is told what it clashes with, so they can pick another time or another room while there is still time to tell people.
