Deduping on TAPE??
I think there aren't too many folks who WILL dedupe your tape data. Think of the huge number of tape mounts to do it, and also to retrieve files. Data Domain will sell you a VTL (diskpool) which again is not tape. Remember we are talking about sequential media, here.
The dedupe works on your diskpools fine. I haven't got any Xfactor numbers yet, but anything free that works is better than nothing. If you are a huge tape vault, with small diskpools, then you won't see any benefit.
Remember that when it flushes the deduped data to tape, (migration, copy, etc) it will "un-dedupe" it (for lack of a better term) so the instances of the file are in all appropriate file spaces.