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You receive notifications of event modifications from a user that is not an organizer or a guest in the event.
Check the Calendar log events to identify the API used. This will reveal CalDAV API to be the culprit.
Sync calendar with the native application only through Exchange as described here.
You also have the option to use Google Calendar app for iOS devices, which is our best recommendation. It is the interface that connects the best with our product. Check here how to use the Google Calendar app for iOS devices.
The notifications and modifications are normally unwanted and created by the iOS native app synced with the CalDAV protocol.