It happens for few..
It is cos of the over concoction of X chromosome to Y.It occurs ancestrally and praise the lord that it is not contagious.You need not worry since it doesn't scathe you in any way rather i should say it is helpful that your blood matches with any other blood groups.A+ and B+ is the rarest combination,about 1/10 mn.Personally,I'm one among that.

Cheers :)

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Pray, Before prayers are said for you..