Triple modular redundancy (TMR)
Beginner
Keeping three copies and going with the majority.
Novice
Building three copies of a storage element or circuit and passing their outputs through a voting circuit that outputs the majority answer, so one wrong copy is outvoted.
Expert
Flip-flop-level TMR masks a single upset but not a glitch that all three copies capture together; full TMR also triples the voters and logic. In FPGAs, tripling everything except clocks, resets and enables (distributed TMR) reduced errors more than tripling only flip-flops in a NASA beam test.
Explained in Space (Industries).