"have you ever seen a game that's been published on Steam and also on another site?"
All the time? Humble store, GOG and even this very website itch.io, which by itself has Brigador, Nuclear Throne, Snakebird, Captain Forever Remix, Sky Rogue and hundreds if not thousands of others which are all also on steam. In fact, many games on the humble store have both direct downloads and steam keys available. Look it up sometime.