Why does every event have two notifications – one from Day by Day app, the other from a default calendar? - day-by-day-calendar/feedback GitHub Wiki

Our app uses the same database as well as a default calendar. When the event reminder goes off, you can see our notification icon, so you can open a dialog box and postpone or turn off the reminder. The same function is fulfilled by an Android-based default calendar.

Android OS security restrictions do not allow an app to change settings of another app. That is why our app cannot turn off the reminders of a default calendar app. Only users can change the settings. If you want to turn off the default calendar reminders, choose “Settings” in the calendar menu, set “Off“ in the option “Customize reminders”.