Power loss while a defragmenter is writing file data will not cause data loss, even on the most basic of file systems like FAT or HFS. Duplicating data into free space is effectively a no-op until the file system is updated to point from the old location to the new location.

It think most versions of windows that included a defragmenter as part of the OS were XP and later, all of which would have used NTFS, which is a journaling filesystem.