How do I prevent getting several alerts for a calendar event?
You'll need to change the notification behavior (disable) in Google Calendar.


On your computer, open Google Calendar.
In the top right, click Settings Settings  and then Settings.
On the left, under "Settings for my calendars," click the calendar you want to change and then Calendar settings.
Under "Event notifications" or "All-day event notifications": 
To edit a notification, choose if you'd like to receive a notification or an email. 
To delete a notification, click Remove notification 


https://support.google.com/calendar/answer/37242?co=GENIE.Platform%3DDesktop&amp;hl=en