It doesn't really make sense. If it recognizes you, then it'll unlock, and if it doesn't, then it won't. In the settings when you add more photos of yourself to improve face matching, you add some pictures that don't look like you, but really is you.
In your hypothetical scenario, when the face unlock doesn't recognize you, it'll add your photo for future learning, but it could even be your friend or anyone else, and it'll add it into the program.
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