I'm not sure if it counts as self-cest, but I've run into polyjuice and time turner h/h/d And d/d/h situations in fic. There is also the fic (and I'm completely blanking on the title, but I'm pretty sure it's by romaine) where Draco is working at Hogwarts and seeing a Harry provided by the room of requirement and then real Harry comes and I think there's a threesome?