So now I have an evil twin. Everyone has an evil twin. And it could decide to come out of the mirror and try to kill me.

[An evil twin that lives in a mirror. That's stretching the limits of how many impossible things he can believe in one day. But, well, warnings of things hiding just out of reach that want to kill him aren't something to be ignored.]

Is just covering the mirrors enough to keep them away?

[Presumably not, if Dorian's twin had come calling. Which begs the question of what does work.]