(You are in the wrong forum, this is the v9.0 forum, but here goes anyway)
That message has one and only one meaning. The tape that BackupExec tried to write to has already been written to by this job.
This usually occurs in stand-alone drives when they fill up, or in libraries that have been partitioned.
if you have scratch/overwritable tapes loaded in the library (they show up in blue if you do an inventory), and if the job is pointed a the library as a whole and not to a partition, then yes, BackupExec should gran one of the other tapes