Is this cheating?

You break up with someone, but they don't accept it or seem to understand that you are no longer dating. If you date, kiss, have sex, with someone else is that cheating. You broke up, it's not your fault that the other person is refusing to accept it.

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This happened to a guy in my highschool. He broke up with his girlfriend, and she didn't accept it. He asked another girl out and started dating her. The girl who didn't accept the end of the relationship was furious.
Is this cheating?
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