In the case that I asked a question, waited a few days, and then accepted a great answer that told me exactly what I need to know. Afterwards someone adds an answer that gives the same important advice in a more complete way with additional insight to which I would have chosen their answer instead originally.
My opinion is that a more complete answer isn't reason enough to dock someone's reputation points when the original answer was given promptly and who's not to say that the second answer wasn't based on the original answer anyway since they more than likely read the accepted answer before answering themselves. I think its great that they are contributing something more and I would up-vote it but am I wrong in not accepting their answer?