Pipeline- only trigger if field has changed

I have a Pipeline that currently works great, but it duplicates the record anytime a change is made. Here's what I have:

Trigger
Record Updated: [Contract Ending]          which is a formula field:
If([Contract Extension Date text]<>"", [Contract Extension Date], [Contract End Date])

Create an Event: (to the selected Google calendar with the fields filled in)

All the [Contract Ending] records are on the calendar now, but it's duplicating the event any time I make a change to ANY of the contract dates since it's a formula. So then I tried Search Events and then Update Event. Then I tried doing a query expression but I'm not sure how to format it. Maybe it needs to trigger from the other date fields?

Basically, I need an event created to a Google calendar, with the [Contract Ending] date, which is either the [Contract End Date] or [Contract Extension Date], depending. Then, IF there is a change (most likely only in the extension field), I want it to update on the existing calendar event.