The duplicating events issue may be related to a sync glitch or to the fact, that Calendars 5 displays the same events from the built-in calendar app on the iPad/iPhone along with your Google account data.
Follow the steps below to remove the duplication:
iPhone
iPad
In this case Google Calendar attached to the iOS calendar app will be removed from Calendars 5.
If you still can see some events duplicating, check the list of all calendars under the icon and uncheck the calendar copies.
If the issue persists, please contact our support team. For this:
A support log will be attached to your email. It will be also very helpful if you describe the situation in detail so our Support team could provide you with a solution.
