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The title is the question: why do obvious duplicate questions on MSE get upvoted, answered, and their answers upvoted? And what can be done to prevent this? Here is an example: https://math.stackexchange.com/questions/576520/does-sum-n-1-infty-frac-1-n-diverge . This question had about 3 upvotes when I saw it, and several people answered it. One of the answers had 5 upvotes! Probably many of the people who answered knew that the question was almost certainly a duplicate. I don't blame the OP, who probably had no idea. I saw the same thing happen a week or two ago when, for the umpteenth time, someone asked how to evaluate $\int_0^\infty e^{-x^2}\,dx$.

I don't think the same question should get answered multiple times on MSE unless someone has a truly original answer, and there are only so many good ways to show the harmonic series diverges. I know I can flag such questions as duplicates, and leave a comment to the OP with a link to a previous question. I can downvote an answer because it is certainly equivalent to a previous answer to the same question. But I think downvoting answers costs me a small number of points, so I'm reluctant to do it.

By the way, there is a related question at: http://meta.math.stackexchange.com/questions/4975/downvoting-because-of-duplication

I had trouble thinking of good tags for this question, so feel welcome to add more or remove the ones I used if you don't think they fit. There is an (exact-duplicate) tag, but I'm guessing it is for people who go fishing for answers to their homework problems by asking the same question several times.

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    $\begingroup$ To clarify: downvoting an answer costs you 1 point. Downvoting a question is free. $\endgroup$
    – Nate Eldredge
    Nov 22, 2013 at 1:29
  • $\begingroup$ @NateEldredge : Thanks. I should now this by now, and it is probably in an FAQ somewhere, but is duplication considered a valid reason for downvoting a question? If I downvote a duplicate question, does it cost the (usually innocent) poser of the question any reputation points? $\endgroup$
    – Stefan Smith
    Nov 22, 2013 at 2:02
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    $\begingroup$ See this thread from last summer. $\endgroup$
    – Jyrki Lahtonen
    Nov 22, 2013 at 7:58
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    $\begingroup$ @Jyrki: Now everybody knows what you did last summer. $\endgroup$
    – Asaf Karagila Mod
    Nov 22, 2013 at 11:07
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    $\begingroup$ A good answer is a good answer, irrespective of whether the question is a duplicate. $\endgroup$
    – Brian M. Scott
    Nov 22, 2013 at 19:31
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    $\begingroup$ @Brian, it's better to have all the good answers to any given question in one place, not scattered around. If you have a good answer to a duplicate problem, close the duplicate and post the new answer at the original. $\endgroup$
    – Gerry Myerson
    Nov 23, 2013 at 6:04
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    $\begingroup$ @Gerry: I don’t feel strongly either way, but I don’t disagree. However, that wasn’t my point. My point is that voting on an answer should be determined solely by the answer and its relationship to the question. There is nothing wrong (and much right) with upvoting a good answer to a duplicate. $\endgroup$
    – Brian M. Scott
    Nov 23, 2013 at 15:49
  • $\begingroup$ @Brian, OK, I misunderstood your comment. $\endgroup$
    – Gerry Myerson
    Nov 24, 2013 at 4:06

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I think the answer is simple: Because people want to earn reputation points, and possibly even a nice golden badge, or at least a silver. A nice strategy in this context is to answer this kind of very popular questions that everybody thought once in their lifetime/career/afterlife. The same goes for the hopefully rare scenario of the questioner with bad intentions: I myself am thinking about resurrecting the randomness of pi and its philosophical implications. I am at least hoping to get a silver badge for my efforts.

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  • $\begingroup$ Thanks. Yes, there is a pretty obvious reason why people answer such questions, but why on earth do others upvote answers that are almost certainly the same as previous answers to duplicate questions? $\endgroup$
    – Stefan Smith
    Nov 22, 2013 at 2:00
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    $\begingroup$ @StefanSmith My guess is that there is a constant accumulation of new users, they do not know the existence of the previous question and its answers, and they are too lazy to check whether the same answer appears in the previous question. Laziness is one of the seven deadly sins, sometimes even papers with no new contributions (or even plagiarised papers) get published as the reviewers and the editor are just too lazy/ignorant to check. $\endgroup$
    – Lord Soth
    Nov 22, 2013 at 2:05
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    $\begingroup$ It's not always about points and badges. Sometimes I'll write an answer because writing a good hint (that I have in mind), or even a short explanation will take me significantly less time than hunting down the duplicate. $\endgroup$
    – Asaf Karagila Mod
    Nov 22, 2013 at 2:08
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    $\begingroup$ @AsafKaragila You are right; that is a possibility that I have overlooked, especially for more obscure questions (I don't think it will be tough to find the question on harmonic series). But sometimes even after I mark as duplicate and give the link, people answer the question - in that case, I see the lust for points and badges as the only possible explanation. $\endgroup$
    – Lord Soth
    Nov 22, 2013 at 2:28
  • $\begingroup$ I'm with Asaf--sometimes it's just easier to post a full answer for the duplicate than to find the duplicate. Other times, I just don't care that it's a duplicate, and just want the fun of writing out an answer. $\endgroup$
    – apnorton
    Nov 22, 2013 at 12:15
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    $\begingroup$ Am I imagining things, or is there a possibility of somewhat different cultures having developed in different subfora? Brian Scott is advocating a very relaxed attitude - supporting it with an argument identical to Asaf's. I learned M.SE ropes when Bill Dubuque was a very visible figure. He was even pressing for the idea of abstract duplicates (if taken to the extreme, this would imply that there should be a single question handling applications of Little Fermat, a single question for integration by parts et cetera). $\endgroup$
    – Jyrki Lahtonen
    Nov 22, 2013 at 21:09
  • $\begingroup$ (cont'd) His thinking may have influenced mine, and that of other members spending most of their time in algebra sections. Even though I never really caught on the abstract duplicate idea, being strict about dups seems to more of an algebra thing here. Or is it? May be my sample set is skewed somehow? $\endgroup$
    – Jyrki Lahtonen
    Nov 22, 2013 at 21:12
  • $\begingroup$ @Jyrki: If a question triggers a recollection of a probable duplicate, I almost always spend some time looking for it; mine has quite often been the first close vote on a duplicate. But if I don’t find it in a reasonable period, or if I decide that there are enough differences — sometimes even just in emphasis — then I’ll write an answer; I don’t at all mind having several answers to similar questions, and I think that abstract duplicates are largely worthless. I don’t look for duplicates just because a question seems likely to have been asked before. $\endgroup$
    – Brian M. Scott
    Nov 23, 2013 at 15:57
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    $\begingroup$ @LordSoth: I don’t consider it laziness: I consider it an intelligent use of an answerer’s time. If you are almost sure that there’s a duplicate, by all means spend a little time searching for it, as I do, but if not, or if a duplicate doesn’t turn up reasonably quickly, writing a good answer is a better use of one’s time. $\endgroup$
    – Brian M. Scott
    Nov 23, 2013 at 15:59

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