... Bunuel (not Sam, in this case). This movie is again about human obsession! The trick using 2 different actresses for one role as stated by another user, is not unique per se, but heightens the surrealism of the movie. And it does work perfectly as to other films that couldn't handle that same weight. Or couldn't convey it to the viewing audience at least!
While it has a slow pace, the movie feeds from it, especially because you have to get a grip of the story- and time-line you have to handle. It is not difficult to follow, but you might need to watch it a second time to really appreciate it :o)