No, Miller's comet is not a real comet. It is a fictional comet created for the science fiction thriller movie "Coherence" released in 2013. The film revolves around a group of friends who experience strange events during the passage of a comet named Miller. The comet's effects on reality and the characters' lives drive the plot of the movie.
While the film incorporates scientific concepts and explores the idea of parallel universes, Miller's comet itself is purely fictional and not based on any known comets in real life. The movie takes creative liberties to construct its story and uses the fictional comet as a narrative device.