Hi guys,

I've just discovered (after a few days of furious Google-searching) the text
of the message that *used* to display for us when we attempted to
double-book a meeting room, but stopped showing last week:

"(Name of resource) is already booked for the specified time. You must use
another time or find another resource."

What makes this message show? Right now, our users don't get warned when
they click "Send" on a meeting request, and subsequently the room declines
the meeting *after* the invitations have been sent to users.

What can I do to make this warning show again when the user clicks Send?

Thanks,
Matt