Bpduplicate has a -id switch that you can use that gives the Media Id. From the Netbackup 5.0 Commands Guide:
-id media_id
Search the image catalog for backups to duplicate that are on this media ID. If the original is fragmented between different media IDs, NetBackup duplicates only the backups that exist on the specified media ID. Backups that span media are duplicated, but not any other backups on the
spanned media ID.