This effect has been around since the beginning of time.
Probably one of the earliest effects used in films.
Simple though it is, there are a surprising number of budding editors who have never realised it can be done very easily in MC.
Not so many days ago there was a question on the Forums about how to have yourself speaking with yourself.
This tutorial will explain the basic concept.
Then let your own creativity can run riot and find all sorts of uses for it.

Good luck!
Douglas