This issue was resolved by Apple in iOS 10.3.3. If you are still experiencing this issue, please update your iOS device.
If you or your users have experienced an issue where calendar invitations are automatically forwarding to attendees multiple times per day, you may be affected by a bug in Apple's iOS Mail app.
Microsoft has identified this is an issue with the implementation of the SmartForward protocol feature of Exchange ActiveSync. This is not an end-user visible feature.
The only workaround provided by either Apple or Microsoft is to use Microsoft Outlook for iOS instead of the built-in Mail app.
For more information, please see this Microsoft Knowledge Base article: https://support.microsoft.com/en-us/help/4014990