jquery FullCalendar - items being duplicated

I asked this question about a month ago and got my answer:

jquery FullCalendar using MVC and JSON

However, now I am seeing duplicates when I switch between months:

My javaScript code is:

    var source = '../calendar/PostCalendarData';
    $(document).ready(function () {

        var events = [];
        $('#calendar').fullCalendar('removeEvents');
        $('#calendar').fullCalendar({
            events: function(start, end, timezone, callback) { 
                $.ajax({ 
                    url: source, 
                    type: 'POST', 
                    data: { 
                        custom_param1: '', 
                        custom_param2: '' 
                    }, 
                    success: function (doc) {
                        $.each(doc, function (index, element) {
                            events.push(element);
                        });
                        callback(events);
                    } 
                }); 
            }

        });
    });

I'm populating the calendar using a stored procedure (located here: ../calendar/PostCalendar).

Is there something I need to do to clear out the calendar for each month?

I'm including the PostCalendar method here:

    [HttpPost]
    public ActionResult PostCalendarData()
    {

        SqlParameter param1 = new SqlParameter("@filterValue", "");
        var calAssignments = db.Database.SqlQuery<calendarAssignment>(
        "exec getCalendarInfo_v2 @filterValue ", param1).ToList<calendarAssignment>();

        List<calendarEvent> calEvents = new List<calendarEvent>();
        foreach (calendarAssignment item in calAssignments)
        {
            string[] splitDates = item.futureDates.Split(',');
            foreach(string s in splitDates)
            {
                calendarEvent calEvent = new calendarEvent();
                calEvent.title = item.name + " : " + item.chore;
                calEvent.description = item.chore;

                DateTime _futureDate;
                _futureDate = DateTime.Parse(s);
                calEvent.start = _futureDate.ToString("yyyy-MM-dd");
                calEvents.Add(calEvent);
            }
        }


        SqlParameter paramDaily = new SqlParameter("@filterValue", "daily");

        var calAssignmentsDaily = db.Database.SqlQuery<calendarAssignment>(
        "exec getCalendarInfo @filterValue ", paramDaily).ToList<calendarAssignment>();


        foreach (calendarAssignment item in calAssignmentsDaily)
        {
            string[] splitDates = item.futureDates.Split(',');
            foreach (string s in splitDates)
            {
                calendarEvent calEvent = new calendarEvent();
                calEvent.title = item.name + " : " + item.chore;
                calEvent.description = item.chore;

                DateTime _futureDate;
                _futureDate = DateTime.Parse(s);
                calEvent.start = _futureDate.ToString("yyyy-MM-dd");
                calEvents.Add(calEvent);
            }
        }

        return Json(calEvents);
    }

I have also verified that calEvents in the C# code always returns the same number. So it appears that it is specifically related to the javascript calendar.