iCal and Google Calendar creating double notifications
To avoid getting double email notifications for appointments, you can turn off the alerts for the Google Calendar on the Mac as described in the Calendar User Guide:


  Turn off alerts for a calendar
  
  
  In the Calendar app  on your Mac, Control-click the name of the calendar [your Google Account name] in the calendar list, then choose Get Info.
  
  If you don’t see the calendar list on the left, choose View > Show Calendar List.
  Select “Ignore alerts,” then click OK.
  


Note that this will turn off all alerts for Google Calendar that would be created on the Mac, that is, not only email notifications, but also notifications in the Notification Center.