This is more than likely due to the filter setting for the Agenda view. Simply open the filter view by tapping on the funnel icon on top right, scroll down, and uncheck “Include Tasks Start/End Date” if it is checked?  We have that option as some users want to show tasks both on due dates and start/end dates, and when you change the status of a task to started or completed, it’s Start or End date is automatically set, which may be causing it to appear twice (once for Due, once for Start/End).