doppelganger contiguity

In a world of 7.9 billion, people constantly don't wanna show their face to each other, why?

They wear mask just not to meet their doppelganger.

in this world, there is a concept that when a person encounter his/her doppelganger they fight to death for his/her existence cause one of the person with same face is said to be fake.

And what happens if the fake one erases the true one existence?