I'm not a medical professional but I have a few years of biology behind me. When a sperm penetrates an egg, the egg sends out a membrane that prevents all other sperm from entering. Thus a child has one set of DNA from the mother and one from the father. Any "extra" sperm leave the body (unless there happen to be two eggs available, thus fraternal twins).
Once a baby is brought into the world (alive or not), that child can have only one set of DNA.
If a woman decides to have a child with another man then that second child's DNA will not match the father of the first child.
DNA evidence does not give 100% reliability (it's based on probablilty, a component of statistics and there are no such things as 0% and 100%) but it gives enough reliability that legal courts accept it.
I hope this helps you.
Good luck.