Kood's 67mm Double Exposure Filter for Cokin A/Snap! lets you create photos with the same subject on both sides of the picture. Half the filter is transparent the other half opaque.
Using a camera with multiple exposure capability, on a tripod, put the dual image filter in the holder and expose half the shot. Then, rotate the holder and move your subject to the other half of the frame and expose the frame again. The effect works best with apertures of f8 or smaller.