First, as others have pointed out, Simpson was acquitted because of a racially biased jury. Second, that was in 1995, and DNA tests have improved over the last 16 years of research. In particular, tests are much faster and more sensitive, and test more of the DNA, thus decreasing the probability of a false match. Thirdly, if you have a DNA match that could apply to, say, 10000 other people, but you have evidence that one of those people was at the scene, you can simply calculate the odds that another matching person was present at the same time.