I'll try and keep this brief, and hopefully include enough information for you to answer the question without you falling asleep part way through.
We recently upgraded our old SBS 2K Server, new box, new OS, new BackupExec. We are using the lower capacity tapes from the old server for differential backups and some new tapes for a full backup at the weekend. We are erasing the old tapes as we are using them, and a new 4mm ID number is being assigned. No problem so far.
However, we have reached a point where there are some old tapes that have the same number as some of the erased tapes (eg 4MM001). When we put these in for the differential backup, will BackupExec spit this out, or will it know that it is not the right 4MM001 (or whatever number it is) and erase it?
I hope that is explained clearly, if not, then I will try again if required.