Q: If I use Windows Server 2012 data deduplication, how much data gets backed up?
A. Windows Server 2012's Data Deduplication feature allows block level data deduplication on NTFS volumes, reducing wasted space. The amount of data backed up depends entirely on the capability of the backup software.
If the backup software is Windows Server 2012 data-deduplication aware, then, for example, if there is 1TB of original data (but deduplicated it only uses 200GB of space) the backup software would only have to back up 200GB of data. System Center 2012 Data Protection Manager SP1 is an example of a data-deduplication aware backup solution.
If the backup software is not data-deduplication aware and just fetches files, then it would back up the un-deduplicated size, which would be 1TB, to use the previous example. The in-box Windows Backup feature is an example of a backup application that isn't deduplication aware.