Some questions, more frequently - some easy questions, usually have a number of answers with only one or two being unique.
Is there any mechanism to clean/detect those or is it only in the hands of up/down votes of community? And if not - shouldn't there be one?
Suppose I post a solution and then I see that someone had already posted the same solution just few seconds before: I would delete mine if I don't see any improvement to do.
I don't think there is more mechanism than each one deciding what to do with his/her answer. Maybe there could be one, but then it would be quite difficult to fix the level of "similarities" to make one be "equal" to the other.
If two answers are identical or one is an absolute sub-set of the other (and looks to be plagiarizing), you should flag the plagiarizing answer for moderator attention, and write why you are flagging it.
Note that this doesn't apply if the answers have arrived shortly (within minutes) after another, or if the shorter answer is much older than the better answer.
But especially if you come across a new answer to an old question in the review queue, and you notice that the new doesn't add anything, then you should flag it for removal.
I'm not sure if such a mechanism currently exists, but to make sure all of the valuable SO knowledgebase is preserved, if a clean-up system is made, that system should somehow merge the questions and not delete them. As an example, yesterday I was searching on very tough issue and I got 11 questions with the similar title. Even though most of them were marked as duplicate of each other, only question #10 contained the answer for my specific problem. I just want to say we should be very careful on deleting information, even if they appears to be duplicate.