A duplex disk is a pair of disks that store identical information. One can read a given datum from either disk independently.
Writing is done by first writing on one disk and then writing on the other.
If a failure occurs during the first write then the second disk is still consistent. If a failure occurs during the second write then the first disk is consistent. Either way at least one of the two disks is in a consistent state at all times. Having seen the UNDO and REDO rules in this simple case, the following formalizes the general rules. The original value is called the pre-image or undo image.