If someone is leading a “double life” it means that in one area of their life they act a certain way and portray a certain image, whereas in another area of their life they act and behave completely differently; hence the term “double life”. Usually the reason that the person needs to portray two different characters is that they have some secret part of their life that they don’t want other people to know about.
As an example, imagine a really nice mother who is a great wife, takes great care of her kids and has a nice job. Everyone thinks she perfect. Secretly however, she dates several men behind her husband’s back and tells no one about it. This woman is leading a double life.
Sometimes teens lead “double lives” because their parents won’t allow them to do certain things that their friends do. They seem like perfect kids in front of their parents, but do all sorts of “bad” things behind their parent’s backs.
Eliot Spitzer is a man who got caught leading a double life.