You’ll usually learn this in Grade 11/12 biology. Basically during the making of the sperm cell (in a process called meiosis) there is a step called a ‘crossover’ where your ancestral DNA will line up with each other (crossing over) and exchange genetic information in a randomizing process to create more different types of sperm (increasing genetic variability or genetic differences). As a result, it is highly unlikely any of those millions of sperm cells will have an exact match because your nature tried to randomize the results to produce more varied offspring to offer a better chance of survival. If everyone had the same DNA, the same disease/weakness would kill all of us, so nature likes to make us all very different if it can to increase the odds of survival.
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