Sometimes you'll come upon a question to which you know the answer, but the question has already been answered. What to do? You should always read the existing answer and think about it. If it's a great answer, you might just move along. But if the answer omits some important details, you might reply and add the missing information. If you think the anwer is wrong, you can reply and say so -- nicely. The important thing is to read the whole thread before getting involved.
And please, don't just repeat an answer that's already been given, especially one that was given days or weeks ago. See DontWakeTheZombies for more detail on this.