Duplicate Jobs

Hi, I have a question that might not be easy to explain but I'll give it a try. I have some tapes from specific media sets that are shortly used because the backups spreaded amongst them and I would like to free some of those tapes by creating duplicate jobs that would concentrate the backup sets on only a few tapes. My question is, if I duplicate a media set that is spreaded across two tapes into another one, will it copy only the backup set I chose from a specific tape or will it copy the begining of that backup set that was written in another tape too? Because if that happens, I'll end up having duplicate data and I don't want that, I just want to free som tapes that have little data written on them, like the end of a backup set,  so I can use them on other media sets I have for new backups. Hope I made myself clear.

Thanks!