She's not her. She didn't die, then come in from another door. She got eaten. Eaten. And then the thing that ate her made another version of her with what it had taken from the girl it killed.
I'm not saying she's not a person, or that she's Carnage, because they checked for that and she isn't, but she isn't Gwen. She's not even a ghost. She's somebody else who remembers the same things and looks the same, and that's worse, somehow, like killing her wasn't enough.