[bsder]

> My understanding is that DNA is basically unique

It is, in a lab environment with lots of source material.

If you are taking small samples of DNA from terribly dirty places and then amplifying the hell out of it, you get massive amounts of contamination.

Rape kits are "mostly" reliable under most circumstances, but have some problematic edge cases. Swabs from crime scenes, on the other hand, are generally garbage.