A standard trope in the genre is a nagging uncertainty about whether the double is real at all. Is this actually an identical stranger, or are they a long-lost twin?

From the Germans, English has obtained a word for it: doppelgänger. Someone walks the earth who is your twin, your alter ego, your shadow, another you, another likeness, who sometimes seems to be close by your side and is your other self. When you talk to yourself, scold yourself, stop yourself up, perhaps you are addressing your doppelgänger, not