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The analysis and synthesis of the fail-safe logical systems have been studied by many authors. They utilize logical elements with asymmetric failure. It is shown that by duplicating two identical systems in parallel it is possible to raise the reliability of the system considerably. If, for example, 0 is the safe value, the duplicate output (0, 1) or (1, 0) is regarded as 1. The output (0, 0) is regarded as 0, which cannot, however, be judged to be correct or to be incorrect.