But is this effect a difficult one to produce? Modern technology has made it very easy to clone yourself on video with even standard home video equipment. Unfortunately it's not really possible to do this effect with the lowest of low end editors (i.e. the free editors like Windows Movie Maker et al.) but it is certainly well within the capability of most medium range video editors like Sony Vegas Pro, which was the Editor of choice for this demonstration.
Taken step by step, this is not such a difficult effect to produce and once the principle is understood, many variations of the basic concept can be created. It basically relies on locking down the camera and producing two identical video clips, one with the actor on one side and one with the actor on the other side. The two clips are then layered, one on top of the other in the editor. Now a virtual "hole" is cut in the actor-less side of the top layer to reveal the actor on the bottom layer next to the actor on the top layer. Because the background to both is identical, both actors appear to be in the same scene.
That's a basic overview of how the clone effect works, but for a more detailed description and examples check out the video below and soon you'll be multiplying faster than rabbits.