Duplex, meaning double or twofold, may refer to various things:

• A duplex locomotive is a type of steam locomotive.
• A duplex lamp is a type of oil lamp invented by Joseph Hinks.
• A Duplex (building) is the U.S. name for a type of housing.
• Duplex (movie) is a 2003 movie starring Ben Stiller.
• Duplex (band) is the name of an electronic music duo.
• Duplex (telecommunications), where a signal can flow in both directions between connected parties.
• The SNCF TGV Duplex is a bilevel version of the French TGV train.
• The Duplex is a comic strip by Glenn McCoy.
• Duplex can refer to a double stranded molecule of DNA or RNA.
• Duplex can refer to a concurrency (road) involving two highways sharing the same roadbed .
• A "duplex reticle" in shooting terminology is a type of crosshairs reticle that is thicker on the edges, tapering to thin in the center of the scope's sight picture. The thicker sections are easier to see against a dark background and the thin lines offer a more precise aiming point, obscuring less of the target.
• Duplex printing is the ability of a computer printer to print on both sides of a page

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