Create Google Calendar Events from Spreadsheet but prevent duplicates

This is very similar to a question asked just two days ago, which was about synchronizing a spreadsheet of events with a calendar. It sounds like you want to consider the spreadsheet to be the master of events that it originates, which would simplify the problem considerably. The basics of what you need to do are covered in this answer. If you’d rather just modify existing code, I’ve got an implementation below.

I have a modified version of the code from this blog, that will modify pre-existing calendar entries to match the info in the spreadsheet. I have arranged my spreadsheet differently, and this is reflected in the code.

Date | Title | Start Time | End Time | Location | Description |
EventID

The event ID column gets filled in by the script when new events are created, and is then used in later invocations to retrieve events from the calendar, thereby avoiding duplication.

Script

/**
 * Adds a custom menu to the active spreadsheet, containing a single menu item
 * for invoking the exportEvents() function.
 * The onOpen() function, when defined, is automatically invoked whenever the
 * spreadsheet is opened.
 * For more information on using the Spreadsheet API, see
 * https://developers.google.com/apps-script/service_spreadsheet
 */
function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Export Events",
    functionName : "exportEvents"
  }];
  sheet.addMenu("Calendar Actions", entries);
};

/**
 * Export events from spreadsheet to calendar
 */
function exportEvents() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var headerRows = 1;  // Number of rows of header info (to skip)
  var range = sheet.getDataRange();
  var data = range.getValues();
  var calId = "YOUR_CALENDAR_ID";
  var cal = CalendarApp.getCalendarById(calId);
  for (i=0; i<data.length; i++) {
    if (i < headerRows) continue; // Skip header row(s)
    var row = data[i];
    var date = new Date(row[0]);  // First column
    var title = row[1];           // Second column
    var tstart = new Date(row[2]);
    tstart.setDate(date.getDate());
    tstart.setMonth(date.getMonth());
    tstart.setYear(date.getYear());
    var tstop = new Date(row[3]);
    tstop.setDate(date.getDate());
    tstop.setMonth(date.getMonth());
    tstop.setYear(date.getYear());
    var loc = row[4];
    var desc = row[5];
    var id = row[6];              // Sixth column == eventId
    // Check if event already exists, update it if it does
    try {
      var event = cal.getEventSeriesById(id);
    }
    catch (e) {
      // do nothing - we just want to avoid the exception when event doesn't exist
    }
    if (!event) {
      //cal.createEvent(title, new Date("March 3, 2010 08:00:00"), new Date("March 3, 2010 09:00:00"), {description:desc,location:loc});
      var newEvent = cal.createEvent(title, tstart, tstop, {description:desc,location:loc}).getId();
      row[6] = newEvent;  // Update the data array with event ID
    }
    else {
      event.setTitle(title);
      event.setDescription(desc);
      event.setLocation(loc);
      // event.setTime(tstart, tstop); // cannot setTime on eventSeries.
      // ... but we CAN set recurrence!
      var recurrence = CalendarApp.newRecurrence().addDailyRule().times(1);
      event.setRecurrence(recurrence, tstart, tstop);
    }
    debugger;
  }
  // Record all event IDs to spreadsheet
  range.setValues(data);
}

Delete / Recreate

In this alternative, the eventID is used to find and delete the previously existing event. After that, a new event is created with the data in the spreadsheet. This has the benefit that all values of the event can be updated, including start and stop times (see Notes below). On the other hand, any changes that were made to the original event will be lost – for instance, if other people had been invited to the event, or custom reminders were added.

To use this alternative, simply replace the matching code with this:

// Check if event already exists, delete it if it does
try {
  var event = cal.getEventSeriesById(id);
  event.deleteEventSeries();
  row[6] = '';  // Remove event ID    
}
catch (e) {
  // do nothing - we just want to avoid the exception when event doesn't exist
}
//cal.createEvent(title, new Date("March 3, 2010 08:00:00"), new Date("March 3, 2010 09:00:00"), {description:desc,location:loc});
var newEvent = cal.createEvent(title, tstart, tstop, {description:desc,location:loc}).getId();
row[6] = newEvent;  // Update the data array with event ID
debugger;

Notes

• The Documentation for getEventSeriesById wrongly states it returns null when no matching event is found, when instead it throws an exception. (nasty!) So I’ve enclosed it in a try / catch block just to keep on swimming.
• Unfortunately, while getEventSeriesById works to retrieve an event, it returns an EventSeries object, which does not support the setTime() method. If you don’t expect to change the time of events, this OK. Otherwise, you can change the Event into an EventSeries by setting the recurrence rules & times, or delete the old event and create a new one, as shown in Delete / Recreate. Issue 1154.
• The spreadsheet always wins. Any event changes (in relevant fields) recorded via the Google Calendar will be overwritten by the script.