In roughly the same sense that Portal is just a clone of Doom.

There are similarities in the mechanism; in the very early game in particular they are nearly identical. But they tell different stories, of different styles and levels of sophistication. They deal with different ideas. And this one goes from start to a satisfying end in a handful of hours.

If you threw in a bit of a science fiction storyline. The game gets more complex, and the goals change several times.