Doppelganger
A doppelganer is an unknown kind creature. Their vary nature makes them hard to study and even dangerous to do so. Little is known about them as a doppelganger is a duplicate of another person, and it would be hard to tell them apart from identical twins. You would also be unaware that you’ve met a doppelganger until you’ve met the person that they are a double of.
Key differences said to be in a doppeler is that they are not identical but look very similar to a person where you could confuse a doppeler for their double, but this is only speculation.
The main danger with doppelgangers is that you could never introduce them to those they are doubled from, to do so would result in one or both of their deaths. It is also unknown as to why this occurs when they meet.
Doppelgangers many not actually be copies but similar people linked by an unknown connection living their own individual and separate lives. Like the study of matter and antimatter, when these two opposing forms of matter meet, they destroy each other.
It it currently unknown as to why doppelers exist, or even if there is one for everyone in the world. It is also unknown if twins are doppelganers of each other in a stable relationship, or if twins have their own doppelers or even if they share a single doppeler.