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From The Collaborative International Dictionary of English v.0.48:
Quadruplex \Quad"ru*plex\, a. [L., from quattuor four + plicare to fold.] Fourfold; folded or doubled twice. [1913 Webster] Quadruplex system (Electric Telegraph), a system by which four messages, two in each direction, may be sent simultaneously over the wire. [1913 Webster]