As far as I understand, you get doubling not because of any interpolation but because you track objects with your eye. That means the tracked object is in rest

As far as I understand, you get doubling not because of any interpolation but because you track objects with your eye. That means the tracked object is in rest in your vision. Which means the screen actually moves relative to your vision. Which means the object smears, where the pixel length of the smear is equal to the pixel distance the tracked object moves in the time the frame is visible on screen. On CRT there is less smear, because the frames are only flashed for a short time on the otherwise dark screen. But if the FPS are not equal to the display Hz, e.g. a half, the tracked object appears on multiple, e.g. two, locations on the screen. Because you show the frame twice, and if you track an object, the screen has moved (relative to your vision) for a few pixels by the time you show the second frame. This happens e.g. if you play a 30 FPS game on a 60 Hz CRT, or when you watch 50 FPS content on a 100 Hz screen. Though the effect isn't usually as dramatic as it may sound here. Doubling is also not as bad as smearing, e.g. in the case of scrolling text.