How to remove Duplicate Event Alerts
- Go to Device Settings
- Select Calendar (or Mail, Contacts, Calendars in iOS9 and below)
- Scroll down to Default Alert Times
- Set Alerts to None
If this didn't help it's most probably an iCloud setting
- Login to www.icloud.com/calendar
- Click on Settings at the left bottom and change Alerts option according to your preferences.
If the above didn't solve your problem, check the following alert settings:
Week Calendar >Settings > Calendar > New event defaults > Default alert
For older app versions see Settings > New Events & Templates > New Event Defaults > Default alert