How can I duplicate myself in a movie (using iMovie)?
There are some good tutorials on this on Youtube here and here. The problem in both of these examples is the obvious lines where you can see the contrast and lightness changes.

However if you had either Adobe Premiere Elements or Adobe After effects this effect is a lot easier, and more professional like this you can find those tutorials here (Premiere) and here (After effects)

Have a rummage around the Creative Cow website for good tutorials