Hello Ian,
The concept of schedule and start times are the same I guess. One is the one that you set in your schedule resource and the other is the time that the job starts running. Usually, they are slightly different.
Again, if you have duplicated jobs starting and you do not want this situation, maybe you need to review your schedules.
It would help us to understand your issue if you could send here the job and schedule definitions. Also, some outputs and log information are always helpful in these cases.
There is a few possible configurations that can force bacula to run a full instead of a differential or incremental depending on changes in the FileSet. So have a look into your configurations would help us to understand this.
The list of files to be backed up is build when the job starts and not when the job is scheduled.
Best regards,
Ana