I am working on a double booking issue where Room calendars appear to be allowing meetings to be booked at the same time as other meetings. The calendars themselves seem to have the correct settings, and I can't seem to recreate the issue myself-- however, I do know it is a problem as this issue has come up once a week for the past 3 weeks in separate resource mailboxes, with different people.

This is an exchange-hybrid setup. Room calendars are created in o365, not synced.

I currently have a case open with Microsoft, however they have been closing the case on me due to not being able to recreate the issue on demand.

Is there anything I could try?

Thank you for any additional information.

Update (10/10/2019):

The Rooms are declining the conflict meetings, but the meetings stay in the user's calendar. Checking sent items a declined notice was sent to the user mailbox.

A room resource not being able to delete a users meeting from their mailbox could be normal functionality, I'm double checking with Microsoft.

Resource Mailbox Details

get-mailbox room@mailbox.com | Get-CalendarProcessing | FL

AutomateProcessing                  : AutoAccept
AllowConflicts                      : False
BookingType                         : Standard
BookingWindowInDays                 : 365
MaximumDurationInMinutes            : 1440
AllowRecurringMeetings              : True
EnforceSchedulingHorizon            : True
ScheduleOnlyDuringWorkHours         : False
ConflictPercentageAllowed           : 0
MaximumConflictInstances            : 0
ForwardRequestsToDelegates          : True
DeleteAttachments                   : True
DeleteComments                      : True
RemovePrivateProperty               : True
DeleteSubject                       : True
AddOrganizerToSubject               : True
DeleteNonCalendarItems              : True
TentativePendingApproval            : True
EnableResponseDetails               : True
OrganizerInfo                       : True
ResourceDelegates                   : {}
RequestOutOfPolicy                  : {}
AllRequestOutOfPolicy               : False
BookInPolicy                        : {}
AllBookInPolicy                     : True
RequestInPolicy                     : {}
AllRequestInPolicy                  : False
AddAdditionalResponse               : False
AdditionalResponse                  :
RemoveOldMeetingMessages            : True
AddNewRequestsTentatively           : True
ProcessExternalMeetingMessages      : False
RemoveForwardedMeetingNotifications : False
IsValid                             : True
ObjectState                         : Changed

  • Is it a particular person submitting the conflicting requests that is causing the issue? Does anyone have access to open the calendar/mailbox directly? – Zoredache Oct 9 '19 at 22:27
  • @Zoredache Some users have said this is not their first time experiencing the issue. – confoundr Oct 9 '19 at 23:28


In according to your issue, I found a document about Exchange online room mailbox double booking, and you can refer to it:


Hope it helpful!

Best regards,

Ivan Wang

  • Ahh ok. This makes sense in this context-- each user who has reported the issue has had the issue with a recurring meeting. The only thing that doesnt make sense is that when I create a recurring meeting my conflicting recurring items get canceled via AllowConflicts: False – confoundr Oct 23 '19 at 16:58
  • No additional users have reported the calendar issue to me. If anything this was closest to an answer as I could get. If it turns out that Microsoft Support uncovers another answer for me I'll update the threads. – confoundr Oct 28 '19 at 16:45


Could you please tell me your account is on Exchange(on-premises) or Exchange Online?

If the configuration is the following, your Room Mailbox doesn’t allow any conflicts:

enter image description here

And I checked your configuration: Yours is no problem, it’s correct.

Best regards,

Ivan Wang

  • Thank you. The Resource account is a resource mailbox hosted in Office 365 (Exchange online). The configuration seemed correct to me as well-- it's strange that the issue keeps occurring though. – confoundr Oct 10 '19 at 14:57

Is there any progress?

I found a thread Double Booking calendars, in according to it, you could try to remove the cachings in Outlook.

(Before performing the method, you could back up these cache files to prevent the loss of data.)

The document about How to clear the Outlook cache: https://www.lifewire.com/delete-outlooks-add-cache-file-1173820

Hope it works.

Have a good day!

Ivan Wang

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