Duplicating events can have several causes. Please check which of the reasons apply to your device.
Things to check first:
- Do the duplicates also show up in the native iOS calendar app?
- Do you have duplicated calendars in the calendar selector?
- Tap the Calendars icon to verify if each calendar has a checkmark next to it
Duplicated Events #1
Your calendars are duplicated under Settings > Calendars, appearing under Local Calendars and under iCloud, Gmail, ... headers.
- Uncheck the calendars under 'Local Calendars'
- In iTunes, stop syncing your 'Local Calendars' in the iTunes settings when your device is plugged in
Duplicated Events #2
iCloud, Gmail, ... incorrectly duplicated all your calendars: under the iCloud, Gmail, ... header, all your calendars show up twice.
- Uncheck the duplicated calendars
- Delete the duplicated calendars on the iCloud, Gmail, ... website.
Duplicated Events #3
iCloud, Gmail, ... incorrectly duplicated all events in the same calendar as real duplicates. You can see this if unchecking (hiding) one calendar removes all duplicates. The duplicates also show up in the native iOS calendar as well.
- Go to device Settings
- Go to Calendar (or Mail, Contacts, Calendars in iOS9 and below)
- In the Accounts section, select your Google account
- Reset / Re-sync the calendar data by toggling account OFF
(You'll get a message "Delete from my iPhone/iPad, this is normal. You can press it).
and ON again
If none of the above solutions work for you, please contact your Calendar service provider (iCloud, Gmail, ...) to resolve the problem.