When duplicate holiday entries are found in the Agenda/Calendar views it is typically a result of having included multiple holiday calendars in the these views. To filter out duplicate “US Holidays”, from the calendar view of our apps, tap on the filter button “funnel icon” (top right corner) and unselect as many of those “US Holidays”/”Holidays in United States” calendar item(s) as you need from the “Include iOS Calendars” section.