[Athlon]

Hmm well the title probably doesnt seem very understanding, but i want to know what the effect is called where there's a video clip with 2 or more of the same person in it.
I will try and put it another way because its hard to explain.

For example, (something i just made up)
There is a camera set up in a room where it can see a desk, bed and the floor. There is a person sitting at the desk. The SAME person appears on the bed and then on the floor. This now looks like there is 3 of the SAME person in the same clip.

I would like to know what that effect is called and how it works. Any ideas? Sorry i dont have any real video examples i cannot find the ones i have seen.

Any ideas will be appreciated.