It means you have more than one backup scheduled for the same time in SuperDuper. While this should be OK, a weird Leopard issue causes more than one copy of SuperDuper! to run at the same time, which generates this error.
If you edit your schedule to set one of the backups to run one minute before (or the others to run one minute after) the others, that'll fix it.
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--Dave Nanian
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