What if I see a question that I know the answer to, but it has already been correctly answered? Should I give my own answer if I feel that my answer is better than the one that has already been provided?
If you can improve the existing answer by editing, without changing their intent, then do so. So, you could improve formatting, spelling, grammar, phrasing, add a screenshot, that sort of thing.
Here's an example of a question that already had an upvoted accepted answer that I thought had a problem. I started to edit it, then realized I would be changing the actual content, and instead posted a new answer.
Kalina has since edited the answer to account for the problem I noticed, but you can see the previous version in the edit history.