No one is 100% identical to their parents. Not even single celled organisms that basically split in half and clone themselves.
Each time a cell divides there are slight changes due to errors and that will accumulate over many generations. Some mutations will have huge effects but most will have slight.
Just to add onto the other answers, a slight mutation rate is actually built into us. It’s *possible* to make proteins (DNA polymerases) that are more accurate at replicating our DNA, but we don’t actually have them because some amount of diversity is good.
For a real-life example, the influenza virus has a relatively inaccurate polymerase, which causes lots more mutations, which is why it’s hard to make a perfect flu vaccine.
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