The phone switch GTD-5 had 2 separate sets of 2 CPUs for each function and instructions were run through both, then compared: https://en.wikipedia.org/wiki/GTD-

The phone switch GTD-5 had 2 separate sets of 2 CPUs for each function and instructions were run through both, then compared: https://en.wikipedia.org/wiki/GTD-5_EAX https://en.wikipedia.org/wiki/GTD-5_EAX "If the results were not identical, the processors were immediately reset, and the pair of processors on the other card were brought online as the active processor complex. The active processor always kept memory up-to-date so that when these forced switches occurred, little data loss was suffered. When the switch was requested as a part of routine maintenance, the switch could be accomplished with no data loss at all."