If someone tells you that you have a doppelganger, it’s not really something to worry about. It just means that there’s another person out there in the world that looks a lot like you! The person could be your identical twin (if you have one), or it could just be another person who reminds people a lot of you.
The word doppelganger comes from German folklore. There were legends and stories that say each person has a ghost-like doppelganger, which is an exact copy of the person. In the legends, doppelgangers are usually invisible. Sometimes, they make themselves seen, and that is a sign of bad luck. Luckily, those are just legends! Usually when people say doppelganger today, it just means a coincidental look-alike.