Don't need an answer, just putting this here for future reference.
Had a user who was sending out multiple "Accepted" and "Declined" responses for her meetings after she had accepted once. The meeting organizers were getting 10-12 responses for each meeting she was a part of.
Traced the issue to the "Outlook App" on her iPhone. "Get-ActiveSyncDeviceStatistics" is your friend. Had her stop using the Outlook App and start using the built-in iOS email. Problem gone!