Is it normal for a guy to be close friends with his ex?

They were together last year for two weeks. It was quick, they got together immediately after they knew each other. But they really liked each other I guess. Then they broke up I don't know why and didn't talk for like 2 months before they started talking again as friends then became close friends. But I kinda see like there's light flirting between them (dont have any proof I just feel the attraction I don't know why) Should I be worried?
Is it normal for a guy to be close friends with his ex?
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