Fairly normal, unfortunately. It's an ugly side of human beings.
I'm assuming you are talking about someone who is nice to you in person and not so nice to you in a group setting. Happens all the time. When we are alone with someone, even if it's someone we can't stand, we are generally very polite and do what we can to get along with them. In a group environment, or when we get away from them, we don't usually feel compelled to be as nice to them.
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