Windows Server Deduplication
Windows Server 2012 and later have a data deduplication feature that is separate- and unrelated- to GBM's own deduplication, that can be used to increase free disk space on NTFS volumes. A scanning process runs in the background to find and merge duplicate file content. By default, the scanning process runs overnight.
Deduplicated files look and behave like normal files; however, they are stored on disk in a special format, that can only be read by Windows Server (and Linux). Non-Server versions of Windows are entirely unable to read these files from disk.
When backing up deduplicated files with GBM, it backs up the full (rehydrated) file content, and then applies its own deduplication to it. This means that Windows Server deduplicated files can be safely restored to non-Server versions of Windows.
When restoring deduplicated files from GBM, the files are restored in their full (rehydrated) format, and are not re-deduplicated until Windows runs its next background scanning pass. This means that you may not have enough free disk space to completely restore a backup to the same source drive.