You always backup to your Dedupe target which is either the Deduplication Folder, or an Open Storage Appliance.
Then when you do a duplicate to tape, the data will be hydrated into its FULL and ORIGINAL data size.
Why do we backup to disk? (forget about Dedupe for a moment) We backup to disk, so that recoveries are faster. Restoring from disk is easily 10X faster than tape or more! Factor in Dedupe, I can now store more than 10X the amount of FULL backups to disk in the same amount of space as without using dedupe. This means that restores going back 30days or 60 days from disk are entirely possible. Saving you a ton of time without having to go to tape.
Now the export to tape that re-hydrates the data, makes your restores faster too! If the data were deduped to tape, it would take hours to reassemble a single file spread across multiple tapes. Not to mention having some staging area for reassembling the data. But in it's full original format, we can now restore direct to the original location as is, no staging nor re-hydrating of data.