There are definitely times when someone chimes in just to repeat a point others have made. Perhaps because they didn't read the other answers.
But repeated answers can happen despite good intentions. Here's some advice if you have repeated an answer, or are tempted to do so because of problems with an existing answer.
The Race (and that precious reputation)
Several users answered the question at the same time. It's tough to delete the thing you spend 20 minutes writing just because someone else only spent 18 minutes to say the same thing, and you think yours is a little better. Plus, yours got some upvotes so you got a little reputation out of it. (The community, then, should not consider an answer posted soon after another one a "repeat" even if it is similar - as they were being written at the same time.)
If this happens, take a look at all the answers. (It does not matter if yours was a little quicker or slower than the others.) Is there a good point you are making that the others don't? If so, expand on that point. Your answer doesn't have to be the all-encompassing, it just has to stand on its own, and add to the topic.
If the answer has no real reason to exist, please do your best to hit that delete button. The reputation will come. (And, if you've recently gotten more reputation, consider deleting some of your older, not-so-awesome answers.)
("The Race" can be very bad over at Stack Overflow, where a quick and dirty answer to a simple question can get you 25 precious reputation from the OP, who is just looking to fix their current problem. It's less of a problem here, but if folks down-vote "repeat" answers that get posted shortly after a sloppy answer that makes the same points, then we could inculcate racing here.)
Brevity
Another question might have answered the question, but left out strong evidence for support.
You might see an opportunity to "win" the question, but you'll make friends better if you first comment on the existing question, pointing out the missing evidence. (And anyone who notices your good teamwork may be more eager to provide comment to improve your answers, in the future.)
Unclear writing
Sometimes an answer answers the question, even though the language is not perfectly clear. Native English speakers might use colloqiualisms or regionally-known references that would leave others confused.
Again, before writing your own improved answer, point these problems out in the commments.
Or edit the answer yourself if you can do so without changing the meaning of the answer. An example of amight be adding a metric version of some measurement. (The rules might use "feet" as a measure, but that doesn't mean all English-language players need to know about the difference between and ounce and a fluid ounce.)
But sometimes, it is appropriate
Here's a recent example (Do rogues get sneak attack damage added to attacks made outside their turn?) of a question that has an answer that repeats a previous one, but (I think) improves it.
The existing answer, though correct, doesn't provide supporting evidence. Some strong supporting evidence (from the game designers) has been published since the question was originally asked and answered.
Also, the existing question was a couple years old, so it is kind of a historical artifact now, and the original poster may be done with it.
It's not a chat room or forum
Stack is a very different place from most other social media site. It's going to be an adjustment for any newcomer to get acclimated to the no-nonsense culture here. The site interface encourages new users to post questions and answers right away, but watching and learning a little first might not be a bad plan either.
