If saving a second calendar invite makes the first one disappear from Outlook, the two files probably have the same ID.

Why it happens

Every calendar file (.ics) has an ID called a UID. When two files share a UID, Outlook treats them as the same appointment, so saving the second one replaces the first. This usually happens when both invites were made from one Outlook appointment, for example by copying it and changing the details.

How to fix it

1. Create each session invite as its own new appointment instead of copying one.
2. Save each one as its own .ics file.
3. Send the new files and ask attendees to add both again.

Certain's two Add to Calendar links

Certain has two kinds of Add to Calendar links. Check which one your registrants are using.

Event Add to Calendar

Adds the whole event to the registrant's calendar as one appointment. Set it up on the form's Confirm page: check Show link to add to calendar, then fill in the Link Text, Appointment Subject (such as the Event Name), Appointment Location, Appointment Body and Reminder.

• The settings on the event's primary form are also used by the Add to Calendar data field in email templates, and for agenda items in the Personal Agenda on the confirmation page. See Why every event needs a primary form
• Keep the Appointment Body short (about 500 characters or less) and link to a website for longer details. Plain text or simple HTML works.

Session Add to Calendar

Adds one session at a time. Set it up on the form under Sessions > Session Display Options. To find it, go to Plan > Forms, open the form and select Sessions. The Session Display Options button is in the button bar at the bottom of the page, next to Help and Preview. It only shows while you're looking at the list of session sections, not while you're editing one.

In the pop-up, go to the Add to Calendar settings: the link text, an optional calendar icon, the appointment subject, location and body (filled from session data fields such as Title, Location and Description), and a reminder.

FAQ

Does this happen with Certain's Add to Calendar links?

This usually happens with invites made in Outlook by copying one appointment, not with Certain's Add to Calendar links. If invites from Certain's Add to Calendar links replace each other, contact Certain Support and attach both calendar files.

It works on my computer but not for an attendee.

Calendar apps handle duplicate IDs differently. Outlook replaces the first appointment; others may keep both. Fix the files so every app treats them as separate.

Next step: Recreate the invites as separate appointments and send them again.