Stop People from Accepting Twice

We use a calendar to post meetings/events/etc. We have people on staff, as well as others that can signup for an event. So I have an Event Calendar table and a second table of attendees - connected many-to-many. The idea being that someone can click on “I’m Going” to add their name, to the event so we have a count of attendees.

The problem I’m having is people can keep clicking on “I’m Going” - driving up counts. i.e. Joe clicks 7 times since he doesn’t remember if he clicked.

I was thinking of adding a unique field to the attendee file and changing to a many-to-one, but discarded that since if an attendee clicked "I’m going’ it would create a new unique Attendee record.

I was thinking of a conditional button, but don’t have direct visibility to the Attendees file to change the state of the button.

I can’t force unique on a connection or any combination of fields - otherwise I could restrict the records in the Attendees file.

I appreciate any suggestions… I’m hoping I’m missing something.