accidental resemblance; chance likeness
A set phrase used when two unrelated people look alike by pure coincidence. Often said with a sense of wonder or amusement.
Those two aren't related, but they look so alike—it's a real case of accidental resemblance.
街で見かけた人が友達にそっくりで、他人の空似かと思った。
I saw someone on the street who looked just like my friend, and I thought it must be a chance likeness.