Looking in the mirror and trying to see yourself is like answering with your name to the question "Who are you?". None of these answers are enough to define yourself. A name and a face do not give you identity, and neither can your age, profession or nationality.
Literature has created the figure of the Doppelgänger, a double, a person who looks just like another, and who can sometimes have or adopt the same name; someone who in theory could replace that person.
Imagine the possibility that someone could take your place? How long could he pretend to be you? For a stranger who knows this double while he pretends to be you the lie may never end, that stranger doesn't know you, he never did, all he knows about you is the acting this double does, the double could start doing things you would never do, and this stranger would never know that he is an impostor. But wait a minute, because surely you have people in your life who know you well, as soon as this double starts acting according to his own thoughts and feelings the lie will be obvious.
But what if the double never acts honestly? What if he is a very good actor? Then it's possible that this person could lie to everyone forever, and while this may seem a bit worrisome or disappointing, that's really when it gets interesting. There will always be someone he can never fool, a person who would identify the lie instantly, and who could even identify the impostor without seeing him and from the other side of the world: that person is you.
Your double could fool everyone, but not you. That's when a new question is born, could you prove to other people who is the real one? Maybe most people can, but it surely won't be by telling their names and showing their faces, because in this circumstance these elements are nothing more than a mask under which a truth and a lie are hidden.
This exercise is interesting, however you may think that all this can only happen in an imaginary scenario, since in real life very few people have someone who could really be a convincing double, and they would not be for long, small but significant differences in experiences and distinguishing factors such as fingerprints could easily allow to differentiate one from the other. For this reason it may seem that this dilemma does not exist for any of us.
But what if I told you that this person really exists? What if I told you that it is possible that this person has already taken your place at some point in the past or could take your place in the future?
Think I'm lying? Stand in front of the mirror and ask him yourself if he is honest with himself and others. If the answer is no, there is your doppelgänger, and only you can remove the mask of the impostor.