Check that the event is not being added from multiple calendars
Remove duplicate calendar subscriptions
Turn off sync on one of the connected devices
Sign out of Google Calendar on extra devices and sign back in
Clear the app cache and data on mobile
Update the Google Calendar app
Remove and re-add the Google account
Check third-party apps that may be creating duplicate events
Verify that imported calendars are not overlapping
Make sure recurring events are not being created twice
Delete duplicate events manually
Use only one primary calendar for event creation
Review calendar sharing settings
Check if email invites are auto-added to your calendar
Disable duplicate notifications or automation rules
Contact Google Support if the issue continues