i know this is from a long time ago, but i just found it useful. i'd also like to add one thing. i did the following:
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removed the cleaning tape/slot
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deleted the duplicate job, then recreated it
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restart the backup exec services (this is the additional step). you can do this by going to: TOOLS - BACKUP EXEC SERVICES, then choose "Restart All Services"
at first it didn't work after performing the first 2 steps. it wasn't until i restarted the services that the error went away and the duplicate job completed.