I'm quite new to FCP and I'm trying to achieve a certain effect..
Final Cut would be where you assemble your footage, not where you necessarily perform the compositing and effects construction and design.
Spend a few hours in the manuals and on google looking up terms like "difference matte" and "locked down effects." Then try some of that advice. Then try looking up "rotoscoping."
We see this question about once a month here and on the Motion and After Effects forums. Once ou find some threads, you may also find some links to production blogs from some folks who have used the technique successfully. Hopefully they will have lots of advice on what NOT to do.
As an effect, multiple appearances by the same individual can be classy and elegant. It has been done often by talented wannabes so it's not unique in its basic form. Your challenge is to come up with something original. But you won't be able to elevate what has has become a cliché until you understand the craft of performing the composites any more than you can create and perform good music before you learn how to play an instrument and play it very well.
bogiesan