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Duplieren ("Doubling") is a sword technique mentioned in the writings of Johannes Liechtenauer, Ringeck, Joachim Meÿer, and others. It is a response to a cut being displaced or parried. The swordsman immediately cuts to the other side (eg. right v. left) without withdrawing his blade away from to opponent's blade.
Meyer describes it as such:
Strike first from your right to his ear, as then when the swords glide together, thrust your sword’s pommel through under your right arm, driving at the same time out with both arms, and hit him with the the short edge behind his blade onto his head.