Double-Booked Meeting Rooms in Office 365 (and how to avoid them)

Update: November, 2017: If you look at newer postings you'll see details on our new method of finding and fixing double booked meetings.  If you want to try it, contact us with the size of your Exchange installation and your test domain (really, please don't start in production!).


Let's talk meeting rooms (aka resources) in Office 365 / Exchange 2016.

Why might you NOT "Allow repeating meetings" when configuring meeting room booking options?

Short answer: Because you have to be really careful.  Especially in Office 365 if you are working entirely from the user interface.

Note that for a one-time meeting where I have the Room 222 booked all day, attempting in Office 365 to book that resource does not even allow it to be seen as an option (this is a REALLY good thing):

But setting up a recurring meeting that overlaps with it easily allows me to book the conference room over a conflict, which might cause me as the meeting organizer some worry down the road.

To avoid this unpleasantness entirely, you need to use Set-CalendarProcessing via PowerShell.

To see what the settings on this resource are:
Get-CalendarProcessing -Identity "Conference Room 222" | fl

to set the ones that deal with conflicts/recurring items:

Set-CalendarProcessing -Identity "Conference Room 222" -AllowConflicts $true  -ConflictPercentageAllowed  80 -AllowRecurringMeetings $true  

Note that here we allow a conflict percentage of 80, which is below the threshold we set when setting up our demonstration.  If you want to allow no conflicts at all via auto-booking: 

Set-CalendarProcessing -Identity "Conference Room 222" -AllowConflicts $false -AllowRecurringMeetings $true  

The Microsoft manual for this goes into some good detail:
https://technet.microsoft.com/en-us/library/dd335046(v=exchg.160).aspx

And as always, be on the lookout for functionality that works in Exchange on-premises but differently in Office 365.

Side note: in our classic Sumatra Utilities for Exchange 2003 we included a double-booking checker, and this would not be hard to do but we hear little demand for it in Office 365/ Exchange 2016.

Addition January 23, 2017:
Folks, this post is insanely popular.  And we're wondering why.
Our initial motivation is to make sure people do not run into this during a calendar migration.  But the number of folks reading this regularly indicates it's a common problem.
Now -- Exchange does a VERY GOOD JOB of notifying a meeting organizer of exactly WHERE conflicts are at the time of booking.  Minimal personal organization skills then allow you to make modifications to individual instances to smooth things over.
Only thing we can figure out is that folks don't do this and they're looking for an upcoming "oops!  You have a conflict in that meeting room coming up!" notice.
Is that the case or is it something else?
We never learned anything listening to ourselves talk, so if you have something to share please let us know.