Both are possible. "A John" = A person named John. A person knocks on the door and says to X: "Hi, I'm John.
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tkacka15There is a John at the door.There is a person named John at the door. (Existential there.)
tkacka15There is John at the door. / There is John — (he's) at the door.John is there, at the door. (Locative there; "in that place".)