Office 365 users report getting multiple meeting response messages.
This issue is caused when iOS 9x users accept meetings on either their iPhones or iPads with the native iOS Exchange Active Sync.
In order to fix this issue, the iOS user (who is sending the meeting acceptance messages) needs to turn off mail and calendar syncing on their mobile device and then turn it back on to stop the flow of the acceptance messages.
On the iOS device - go to Settings - Mail, Contacts, Calendar - Select the account - turn Mail and Calendar Sync Off - Choose "Delete from My Phone".
Wait a few minutes then turn the Mail and Calendar Sync back On. Note: It will take a while for everything to sync again.
Our general recommendations are:
When you receive a meeting invite wait a few minutes before accepting or processing it. Allow time for all your devices (phone, desktop, home) to get the message synced to them.
Do NOT accept/respond to meeting invites and updates from your mobile phone or device. Wait until you are at your desktop or log into the web client at office365.uiowa.edu to respond from there.
For a person doing the scheduling: do not create meetings on mobile devices (personal appointments are ok just don't invite others). Use a desktop client or web client to schedule meetings. When an occurrence of a recurring meeting needs to be moved or updated, it is best to cancel the occurrence and create a new one time meeting on the correct date. Also, don't schedule recurring meetings for longer than 6 months. Start a new series instead.
For more detailed information on Calendar Bes Practices see these articles:
Exchange Calendaring Issues: Best Practices for Outlook and Mobile Device users & their Delegates, Best Practices for O365 Calendar Users.
You might consider using the Outlook mobile app instead of the native calendar app as it is reported to fix the calendaring issues between iOS and Office 365.