There are two answers here. They are both correct and should be joined, but what are the official rules to this? Can the two answers stay there, or should the second one be a comment?
In this case hieu.do's answer wasn't an answer but rather a comment on the existing answer, so I made it so.
In the general case, if a question has multiple answers that are essentially the same, no action is needed. One answer may be better worded or explained or more convincing than the other, then you'll upvote that one. One answer may be a crappy oneliner, then we should either edit it or delete it — but do so because it's a crappy oneliner, not because it's a "dupe answer."
If one answer is great and the other's merely good, then the latter doesn't need to be flagged or downvoted. If it's an okay answer but it's not great, just don't upvote it.