The title of the movie is "Mewtwo's Awakening". It should be obvious that the old Mewtwo, which Ash recognizes in the trailer, will be the focus. It could easily change its Forme to save the day; there doesn't need to be a twin or a replica.
Likewise for the games. Awakening could be the theme of the new Forme, but we would have to actually understand Mewtwo's in-game past for that to be effective.