You work as a security guard to catch doppelgangers and differentiate who's your neighbor to let them in. You stayed in a office, reviewing people behind a glass, and calling the DDD if a doppelganger shows up.

One day a milkman comes and shows he's ID through a slip in the window. He looked tired and his voice was unamused but soothing, "Can I come in?"

You look at his file, see if he's on the list, his name Francis, and then the ID he turned in, he had blood on him, which he swears up and down is scarlet milk.