Lockstep
Beginner
Two identical processors run the same program at once and keep comparing answers.
Novice
Running the same software on two identical processor cores at the same time and comparing their outputs every cycle. Any mismatch means one of them has a fault.
Expert
Dual-core lockstep (DCLS) runs the second core a few cycles behind the first, so a common-cause disturbance hits them at different points and produces different errors that the comparator catches. Standard for ASIL D control functions.
Explained in Automotive (Industries).