When I posted my multiplicity photos in flickr, I was asked by a few guys on how I did mine. Well, I always reveal my photoshop secrets when politely asked and whenever I have the time to make a tutorial. Multiplicity photos are simply made by cloning the same person on the same frame.
How do we do that? We start by getting a stable tripod where you can fix and attach your camera which will take multiple exposures of the same subject appearing on different location in the same background. For this, you should likewise choose a not so busy background or a quiet static place where there is no movement of anything on the background. Doing it on outdoors where there are passing vehicles or branches or leaves of trees swaying would not at all get the effect you needed. Remember that the camera must remain fixed and stationary during the entire photoshoot and you can avoid camera shake or movement by using a remote shutter release trigger or activating the timer switch of your camera.
Start taking pictures with your model moving to a different location each time. The more frames to combine, the harder it will be when you're cloning them in photoshop so in my example, I shot only two frames of the same subject on a different location in the background. Using a wide angle lens in these kind of shots would be most helpful so that you can cover more space in your frame. Also bear in mind that the subject should not occupy the same space occupied in the previous frames, otherwise, an overlapping subject would be difficult to deal with in photoshop. That's about it for the photography part and the next step would be the post editing work.
In photoshop, open the two frames or so much frames that you have taken with the same background. In my example of two frame shoot, you're going to select the second frame using the rectangular marquee tool then move that selection on the first frame using the move tool. While working on the second frame which is the top layer, create or add a white layer mask. Set black as the foreground color and white as the background color. Using the brush tool, slowly paint over the portion where the subject on the background layer (first frame) is located. Voila!! that's about it. You now have a multiplicity photo. Flatten the image and save it!
Another alternative would be using the eraser tool instead of the layer mask technique. I personally prefer the layer mask technique more to my liking but in photoshop, you can always get what you want in so many ways or means possible.