That is correct - when you duplicate a tape, you actually duplicate the images on the tape. So, in order to duplicate the backup image, you need all the tapes in the backup set.
You say 'I have never been able to see this through to completion' - please elaborate? Are the duplicates failing or just taking too long?
Please also let us know which NBU and O/S version of your master/media server. There are TechNotes about failures on large duplicate jobs.