I have a scheduled backup that should run once per day, but runs twice. I've scheduled the job to run at 6:30 AM, daily.
If I view the log of that maintenance plan, I can see that it ran today at 5:00 and at 6:20.
If I view the Job History log, it shows that the backup was run once, at 6:20 only.
This job was scheduled to run at 5:00, but was rescheduled to 6:20. I deleted the 5:00 schedule and replaced it with the 6:20 one under the maintenance plan. I've confirmed that only one job is added for this under SQL Server Agent visually under SSMS as well as querying the sysjobs table and the sysjobsteps table.
I'm not sure what the next step might be to track this down is.