"Drinking and hangover can cause temporary double vision, especially for those who drank more than they should these holidays. But double vision can be also achieved with the effects of Photoshop - example 1 and example 2. I don't know about you, but I feel a virtual hangover just by looking at any of those pictures and my brain starts going nuts. "
For example,
The key for this photoshop effect is that "Do not double a nose as it will ruin the effect which comes from our brains refusing to accept that part of the face (eyes & mouth) is doubled, but part (the rest of the face including nose) is not."
More
http://www.freakingnews.com/Double-Vision-Pictures--1762-0.asp
