Yep -- we have the new Sumatra Double-Booking cmdlet re-written for Modern Authentication.
And it's field-proven,
After you've gotten your IDs and Secret, and loaded the cmdlet, you can have notices like this automatically sent.
One of our tenets for best practices about double-booked resources: Start off by treating your users like adults and letting them work it out. If that meets with inadequate success, ratchet the capability up from there.