What's the point in answering an already answered question?

Question

I have noticed that there is the common habit of answering an already answered question (if the answer is not already accepted). But what's the point in doing that?

If the previous answer is incomplete, I thought I should comment it to make the author editing it or to add any missing point. Instead, most of the people here write a new answer, which content is the same of the previous one but with a couple of clarifications or more in depth analysis.

Is this normal practice? Or it would be better to comment if the answer is not adding any significant point? And moreover, should we down-vote this "copy" answers?

Answer 1

If you think you can write an answer that is more complete, clearer, more rigorous, better supported by evidence, or that otherwise improves on an existing answer then you should do so. You should do this even if there is some overlap with a pre-existing answer.

The purpose of this site is to ensure that the best possible answer for each question gets posted. Absolutely the best way to achieve that is to have people posting improvements on existing answers when they see the opportunity to do so. In fact, the behaviour is actively encouraged by the stack exchange networks, which uses the number of answers per-question as a metric for evaluating the success of the site.

The whole reason for having a voting mechanism on the site is so that the community can choose the best answer and drive it to the top so that it is also the most visible. Nobody will feel offended if you try to improve on an answer they wrote (because everyone here understands that this is a collaborative effort to produce the best economics resource that we can). Unless an answer is an exact duplicate (in which case it should be flagged as spam), each answer should be evaluated on its own merits. An answer should be down-voted only if you think it is wrong, misleading, or fails to address the question. An answer should be up-voted if you think it does a good job of answering the question. If there is more than one answer that does a good job then you should up-vote all of them.

Answer 2

I have had answers that I perceived as very similar to my own posted a few times. I sill don't think downvoting the 'rival' answer is justified. Sometimes the phrasing is different, or the clarification and extra point could be a significant improvement. If it is just a simple copy: People can see the time of posting on both answers.

Let the OP and the wisdom of the masses decide. Seems to me that this usually works 'in a fair way' and the few times it doesn't it does not result in a serious systemic problem.