How accurate is DNA evidence?

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Generally, DNA is 99.9% reliable. In practical terms, in a city of 1 million people, if a person commits a crime and is identified using DNA, only 1000 other people could have committed the crime.

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Everyone accept identical twins would have different DNA. There is also a RARE Genetic disorder that causes a person to have two different sets of DNA. It is called Chimaerism.

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Despite popular belief, DNA evidence is not as accurate as people might assume. The 99.9% people often cite is actually just an estimate and no one is actually sure how accurate it might be. While every person's DNA is different, a DNA profile, the bit used as evidence, is only a small sliver of a person's entire DNA, and even siblings may share a very similar DNA profile. In 2001 Kathryn Troyer ran a test of Arizona's DNA database of felons and discovered two felons with DNA profiles where 9 of 13 markers were identical, despite the fact that one was white and one was black, and later discovered dozens of other similar matches, yet the FBI estimates the odds of some one sharing those genetic markers to be 1 in 113 billion. The fact that only 13 markers are used out of the entire DNA strain should bring into question the validity of DNA evidence.