Marcel Duchamp appears as a signatory to the First Papers of Surrealism, although he wasn't in his day and certainly isn't today, officially aligned with the movement. It's tough; nobody works in a vacuum, and associations are only natural. And of all the movements of his day, Duchamp may have been drawn to Dada and Surrealism, although he was alwasy going his own way . . .
The document (I love 'Explorers of the Pluriverse') includes many who wouldn't now be necessarily thought of as surrealists, such as Edward Lear, and Marc Chagall. But what's typically splendid about Duchamp's input is that whereas Andre Breton is the curator 'hanging' this printed exhibition, Duchamp is deliciously described as 'his twine'.
A full scan is available at First Papers of Surrealism.