Disk duplexing is simply a mirrored pair with an additional adapter on the secondary drive. This provides fault tolerance for both disk and controller failure. (The use of multiple adapters connecting to one drive is not supported.) In addition to providing fault tolerance, this can also improve performance.
Like mirroring, duplexing is performed at the partition level. To the Windows NT operating system, there is no difference between mirroring and duplexing. It is simply a matter of where the other partition can be found.