Quite often while using the new review feature, I find new users who answer questions that already have accepted answers. As new users don't usually have any experience on how Stack Exchange sites work, they tend to answer the questions quite poorly.
What I am suggesting, is a popup message that will appear when they click to post an answer that will say something like:
This question already has an accepted answer. Only answer if you answer is more detailed than the one that is currently accepted.
I know what we tend to do with these posts is ignore them, however this is a really simple change that can hopefully make reviewing new users' posts much easier as we don't have to contend with poor answers posted on already answered questions.