How to avoid fake people?

This guy in my HS is pretty nice to my face (sometimes making little digs) but is very rude behind my back. He goes out of his way to say awful things about me for no reason. Why does he do this while being nice to my face? Once he was having a convo with me and then he leaned over to whisper something rude in his friend's ear. How do I avoid fake people like this and pick up on it sooner?
How to avoid fake people?
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