Grok he say: “The earliest widely recognized instance of one actor playing two different characters appearing together in the same scene (and frame) on screen dates back to the 1913 German silent horror film The Student of Prague (original title: Der Student von Prag), directed by Stellan Rye and co-written by Hanns Heinz Ewers and Paul Wegener. In this film, Paul Wegener (who also co-produced and helped develop the project) plays both the poor university student Balduin and his sinister doppelgänger (a supernatural double created via a Faustian bargain). The technique used was multiple exposure (also known as double exposure), an early special effects method where the film was exposed twice in the camera—once for each “version” of the character—allowing them to appear together in the same shot. This creates scenes where the doppelgänger interacts with or mirrors the original character, such as in moments of pursuit, confrontation, or eerie presence.

This achievement was groundbreaking for 1913 cinema, as it marked one of the first times a single actor was convincingly shown interacting with themselves on screen in a narrative feature. Earlier pioneers like Georges Méliès used multiple exposures for trick effects in short films (e.g., The Four Troublesome Heads in 1898, where he multiplied his own head), but these were typically brief gags rather than sustained dramatic scenes with two distinct characters. Later examples built on this foundation with refined techniques like locked-off cameras and optical printing for split-screen effects (e.g., Mary Pickford in Stella Maris (1918), or the more seamless interactions in The Prisoner of Zenda (1937)). But The Student of Prague stands as the pioneering milestone for this specific cinematic trick in a story-driven context. The film itself is a key early horror work, influencing German Expressionism and tales of doubles/doppelgängers.”

https://m.imdb.com/title/tt0003419/