There is no double vision for your finger. The actual representation of your finger is the (single) red one, below. You get two fingers shown in the background, because of the way these cameras work. You can think of these as shadows of your real finger.
The device projects an IR pattern onto the scene and captures it, then. So one shadow comes from your finger blocking the projector (so no IR patter is behind your finger) the other one comes from your finger blocking the view of the camera (it can see no pattern behind your finger). Since camera and projector are mounted at an angle, you end up with two shadows.
Explanation of the camera:
https://www.youtube.com/watch?v=uq9SE...