If you have installed the Google Calendar on your Samsung Galaxy S7 because it offers you more options than Samsung's S Planner calendar, then you may always have a double notification at an event. One from the S Planner and one from the google calendar.
Since the S Planner cannot be uninstalled from the Samsung Galaxy S7, please proceed as follows: Open the app menu on the Samsung Galaxy S7 and then the settings. Scroll down to "Notifications" and select the entry.
Then go to "Advanced" in the upper right corner. Let's see the extended notifications in Android. Scroll to "S Planner" and tap the entry. Disable all sliders for the S Planner here. Then tap on "More Settings". Slide the controller from "On" to "Off". Done!
Afterwards, you will not be informed about new calendar events anymore by the App S Planner, but only via Google Calendar. The duplicate notification is not required by the S Planner calendar.
You now know how to disable the double calendar notification from S Planner and Google Calendar.