When leveraging Deduplication, your backup target must be an NTFS disk. You cannot deduplicate to tape directly, at least not yet.
The reason for this is that, deduplicating to tape would increase your restore time, and potentially increase your failure rate to restoring data as tapes go bad more often than disk.
Restoring from a disk target is often faster in every which way possible, and with deduplication, if backing up 2TB of data, and saving to a 3TB volume, you could hold some 2 months of FULL backups to be able to restore from in seconds. Try to go back to tape, which may or may not be in your library (find tape, recall from ironmountain, etc, then inventory) Hopefully it's still in the catalog... Then restore, but the restore job spans two tapes... Not fun.
Ultimately, what is more important, backing up or being able to restoring? Being able to restore directly from disk is the right answer.