Hi Tim,
Does this issue still occur if you 'future date' the next scheduled send to the next day forward (scheduling this change and email addition after the send for the current day has already occurred)? I believe the multiple sends are occurring due to the edits and the scheduled send time being maintained as daily from the current day onward. Since the send has already occurred once that day if you 'future date' by one day that should resolve since you are scheduling for a time that has not been reached yet (emphasis on as long as that time has not passed).
Alternative workaround would be to re-schedule the job from scratch since this assuredly would not have any memory of recurrence and only start from the fresh first scheduled start time in the future.
Best,
Brian