Green’s Dictionary of Slang

a duplicate, e.g. key, identification card; also attrib.

1891		J. Maitland Amer. Sl. Dict.
1916		E.W. Sargent Technique Photoplay (3rd edn) 361: Dupe, a pirated print of a film. Made by passing a genuine positive print through a printer in company with negative film whereby an imperfect duplicate of the negative is obtained from which prints are made [OED].
1930		Amer. Photography 24 440: If the amateur feels that a film will be of great interest in the future, he should have a dupe made before the original ha? been projected more than a few times.
1949		Life 25 Apr. 19: You tell me the reason for the cancellation is the article in Life and the printing of my picture as a ‘dupe’.
1956		J. Thompson ‘The Flaw in the System’ in Fireworks (1988) 134: All our records were made in duplicate, the dupes going to the home-office store.
1960, 1975		Wentworth & Flexner DAS.
1979		‘Iceberg Slim’ Airtight Willie and Me 112: She slipped the dupe key into the pocket of his cashmere overcoat.
1986		R. Campbell In La-La Land We Trust (1999) 190: It’s reserved for Mr. Cape, but I can run you a dupe.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 33: Guy scored him a pass. He didn’t need it. Some geek sold dupes.
2021		J. Ellroy Widespread Panic 22: They sold dildo dupes of Donkey Don’s sixteen-inch whanger.