I suggest that a push-back email "You room booking request is REJECTED" (when the room is already taken by someone else) needs to be marked red. As of now it looks no different from the "Your room booking request is ACCEPTED" email and people do not usually notice that "system" emails at all and therefore do not notice that the booking didn't go through. Even thought the room booking is rejected, it still remains in the work calendar, that's why people do not bother double-checking and do not expect any troubles. Double-bookings occur quite often because of that and it creates a lot of confusion, especially if large groups of people are involved.