Hi Monica
If I understand right you are wanting to receive one cancellation notification when users seminar waitlist request expires. Currently you are receiving the booking cancellation and waitlist signup expired notification.
Each notifications can be disabled at the seminar level. - if you disable the booking cancellation notification then a user not on the waitlist who cancels wouldn't receive the notification.
Disabling the waitlist signup expired notification and enabling the booking cancellation notification would mean just the one notification would be sent
regards