I don't think i've ever seen duplication processes ever actually show correct data in any way, whether its 0 KB or -1,348,567,214% complete. I would assume it would be actually difficult to show the "progress" bar for that based on the fact that progress is determined from a previous backup of a client and since multiple clients are probabbly on that tape, then you'd not show that correctly.
However, without asking the developers why, theres really no direct answer- just something you have to deal with.
The only thing i can say is i've never had duplication take longer than the backups.