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Lately, I came across several suggestions for deletion of questions simply because they are duplicates. In my opinion:

  • If the question is somehow a bad question, then it ought to be deleted, and so should the original one.
  • Otherwise, there is no reason for deleting it. Being marked as a duplicate is enough.

I would like to know whether someone thinks otherwise and, if so, why.

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    $\begingroup$ If it's a duplicate, what reason is there for keeping it? $\endgroup$
    – Gerry Myerson
    Commented Sep 30, 2018 at 11:58
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    $\begingroup$ @GerryMyerson As mentioned a few times here on meta, duplicates can be useful. If nothing else, they might help when searching. $\endgroup$
    – Martin Sleziak
    Commented Sep 30, 2018 at 12:22
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    $\begingroup$ @GerryMyerson If there are no answers, they are useless and can be deleted. But they can have answers which are different from the ones of the other version of the same question. $\endgroup$
    – José Carlos Santos
    Commented Sep 30, 2018 at 12:30
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    $\begingroup$ If the duplicate is an inferior version of the original, by which I mean, stated as a problem statement, showing little or no effort, while the original is a better stated version of the question with effort and/or context, there is no reason to keep the problem statement duplicate, regardless of answers. Note that particularly with poorly stated questions, and on top of that, questions that are dupes, answerers need to show restraint in answering "poor questions*, as well as take responsibility for a quick search to avoid answering poor quality dupes. $\endgroup$
    – amWhy
    Commented Sep 30, 2018 at 15:38
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    $\begingroup$ @amWhy What I had in mind was the situation in which the quality of the duplicate as about the same as the quality of the original question. $\endgroup$
    – José Carlos Santos
    Commented Sep 30, 2018 at 15:39
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    $\begingroup$ Not in those cases where a question is both a dupe and could have also legitimately closed for another reason, too. There is such a phenomenon as a "crappy" duplicate of a far better post. Those should go, with or without answers. $\endgroup$
    – amWhy
    Commented Sep 30, 2018 at 15:54
  • $\begingroup$ Sorry, @MartinSleziak, I meant to ping you in my last comment. $\endgroup$
    – amWhy
    Commented Sep 30, 2018 at 16:01
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    $\begingroup$ "If there are no answers, they are useless and can be deleted." I don't think this is right, as Martin mentioned before, they can be useful when searching. $\endgroup$
    – user486983
    Commented Sep 30, 2018 at 19:05
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    $\begingroup$ For instance consider these duplicated questions. a). Convergence of an infinite product $\displaystyle \prod_{k=1}^\infty (1−\frac{1}{2^k})$ ? [duplicate] and a'). Is the product $\displaystyle \prod_{k=1}^\infty \frac{2^k-1}{2^k} $ necessarily 0?_ And suppose the duplicated question has no answers and also no effort. Even still they should remain in the site because it would clearly make it easier to look up a question in some way or another $\endgroup$
    – user486983
    Commented Sep 30, 2018 at 19:08
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    $\begingroup$ @Isabella I agree. I hadn't thought about that use concerning searches. $\endgroup$
    – José Carlos Santos
    Commented Sep 30, 2018 at 21:49
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    $\begingroup$ Unclear of how to vote given the phrasing of the title! I want to express "duplicates do not normally have to be deleted." To delete them, there would have to be some serious reason for the post to be unacceptable. Otherwise, it can be turned into a signpost. $\endgroup$
    – rschwieb
    Commented Oct 1, 2018 at 16:59
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    $\begingroup$ I tried to express precisely the idea that “duplicates do not normally have to be deleted.” I am sorry if I did not express myself clearly. $\endgroup$
    – José Carlos Santos
    Commented Oct 1, 2018 at 17:43
  • $\begingroup$ @MartinSleziak IMO the linked thread discusses those cases where the new versions asks about some theme using different words. At best, it is unclear what the voters back then thought about those duplicates you can find in a few seconds by copy/pasting a line of TeX into Approach0 search box. May be the OP can clarify? $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 6, 2018 at 11:48

3 Answers 3

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If a duplicate will improve search then it should be left. There are questions on the site whose title bears little relation to the general form of the question it answers, or to a number of different ways of asking, but nevertheless constitute a canonical answer.

As a result, it is next to impossible for users to find that answer and users ask it again, often wording their title differently.

The duplicate question functionality exists for precisely the reason of signposting users who may have searched anticipating a title in the form of the duplicate, to the canonical answer.

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  • $\begingroup$ No, duplicates should be deleted in the common case that they add nothing novel (if they add some text that helps searching then such text should be added to the dupe). Otherwise we end up with hundreds of dupes to common exercises - most being fgitw low-quality answers - making it impossible to search for the "best" answers. Else the site devolves into an ephemeral stream of low-quality fgitw answers instead of the design goal: a library of easily searchable high-quality answers (hopefully evolving into "proofs from the book"). $\endgroup$
    – Bill Dubuque
    Commented Aug 25 at 17:22
  • $\begingroup$ If the site devolves into an fgitw stream then all our prior work is effectively lost, since it it becomes too difficult to locate the best answers among the hundreds or thousands of dupes. Unfortunately most users don't realize this is already occurring since they rarely if ever search for answers / dupes. $\endgroup$
    – Bill Dubuque
    Commented Aug 25 at 17:25
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    $\begingroup$ Tip Whenever you post an answer, always check to see that you can locate it by common searches. If not, then add some keywords or phrases that will aid searches. $\endgroup$
    – Bill Dubuque
    Commented Aug 25 at 17:30
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No. If the original question is a bad question on its own merits, then of course it should be deleted, whether that's recognized today or long ago. Then technically, the duplicate no longer has an original to be a duplicate of.

But if the original is a good question, let's give the duplicate asker the benefit of the doubt and assume that they tried searching but for whatever reason could not find the original.

In my opinion, only a word-for-word duplicate should be deleted (e.g., if both the original asker and the duplicate asker are quoting the same textbook exercise).

The textual differences between the original and the duplicate might be enough to encourage a previously unthought of approach to the problem. At the very least, the duplicate brings attention to an old question that might languish forgotten.

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    $\begingroup$ Exactly, only a word-for-word duplicate should be deleted (supposing both doesn't have good answers). $\endgroup$
    – user486983
    Commented Oct 6, 2018 at 22:32
  • $\begingroup$ I agree that a newbie asker may post a duplicate simply because they didn't know how to use a search engine (or didn't know about Approach0). I am criticizing the answerers who should know better. Also, what I said does not apply to all the dupes. Hmm... looks like I need to be more verbose. Editing my answer. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 7, 2018 at 3:21
  • $\begingroup$ @Jyrki For what it's worth, I upvoted your answer before I posted my answer. $\endgroup$
    – Robert Soupe
    Commented Oct 7, 2018 at 3:25
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As a rule I am in favor of deleting duplicates. Admittedly this is a relatively recent change of heart. The reasons for my change of opinion are the following.

  • Nowadays we have several human answering machines who simply refuse to search before posting their answers - often their answers are also essential duplicates of the old posts.
  • This practice makes the site more difficult to navigate and therefore less useful for future visitors.
  • This is also related to general quality control problems plaguing the site. There is a significant overlap between the set of users who answer low quality questions and the set of users who don't check for dupes.
  • Therefore I want to disincentivize the practice of dupe-answering. Merely closing the question as a dupe won't erase the wrongfully gained rep (=the incentive to increase site entropy), but deletion does work. In other words, only deletion is a sufficient deterrent encouraging A) the askers, and B) the answerers to search first and only then post.
  • Observe that deleting the new version of the question does not stop anyone so inclined from adding a new answer to the older version. Whenever an answerer can add a new angle they can do so easily - deletion does not stop that! They can post their answer to the older version of the question! That action will even bump the old thread allowing voters to see the new addition and evaluate it! I don't see any reason not to post the answer to the older thread - unless the answerer is specifically after that green checkmark from the asker of the new version.
  • As an alternative to deletion we have the mechanism of merging two variants of the same question. Unfortunately this is not always optimal. After all, two variants of the same question may use slightly different notation, and naturally the answerers often copy the askers notation, raising post-merger compatibility issues.
  • There are also those cases when the new version of the question is decidedly better than the older version. May be the newer version is slightly more general, or the fresh asker added more context? In those cases we sometimes close the older version as a dupe of the new one.

Judging from the other answer there may be a misunderstanding. Also, my thinking is colored by those questions that are "obvious" dupes. I put the obvious in scare quotes, because its meaning varies from person to person. Largely addressing users who have spent enough time on the site to earn, say 20k of rep, or who have held an academic position (implying that they have taught a section or three of calculus and/or intro to abstract algebra). Such people cannot possibly think that certain kinds of questions would NOT have been handled on our site ages ago. And I'm not talking about questions themed like "irrationality of $\sqrt2$", "$0.999\ldots$". At the other end, there are truly accidental dupes, I'm not talking about those. What I have in mind are questions like:

  • Find the indefinite integral $\int x\sin x\,dx$.
  • Show that $2^n>n^3$ for $n\ge10$.
  • Stars and bars, edition #117.
  • $x^p-x-a$ is irreducible over $\Bbb{F}_p$.
  • $\cdots$ (you can find such examples from all the undergrad courses and core 1st year grad courses)

That is, questions that appear so frequently in relevant courses. Yes, they are fine exercises to a noob. They demonstrate certain techniques to a student just fine. But, anyone who's seen them KNOWS that they are soooo standard that there's snowball's chance in hell that we haven't already covered them on this site. I'm talking about questions that smell like dupes, talk like dupes, and walk like dupes. MAY BE THEY ARE DUPES! LET's CHECK BEFORE ANSWERING!

I don't have any qualms about somebody who wants to add a new proof to an old problem using a different idea, like here. What I'm maintaing is that people who

  • should (from their experience either in real life or on this site) suspect that this question has been handled here earlier, and
  • yet post a copy/paste textbook answer without searching

are guilty of either sloth or greed (for rep points).

I am not saying that all the dupes should be deleted, but I decidedly oppose the logic that because a dupe may be ok in such and such a situation, we should never delete dupes.

I guess we are back to the old rule of

Judge these matters case-by-case.

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    $\begingroup$ Some of these "answering machines" even duplicate their own answers sometimes. I'm not even sure they're aware of it. $\endgroup$
    – Arnaud D.
    Commented Oct 5, 2018 at 20:01
  • $\begingroup$ In case it is not clear to all the participants, I have a long history of being rather militantly against duplicates and other material that does not add anything useful to the site. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 6, 2018 at 11:55
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    $\begingroup$ There's also the possibility of the existence of two versions of the same question such that each one of them has an excellent answer which was not provided to the other one. I think I've already seen that, but I don't have an example at hand right now. $\endgroup$
    – José Carlos Santos
    Commented Oct 6, 2018 at 13:39
  • $\begingroup$ @JoséCarlosSantos Not denying that as a possibility. My attitude mostly comes from the posts where the askers obviously didn't search. The onus is on the answerers to check. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 6, 2018 at 17:20
  • $\begingroup$ Here is an example of the kind you probably had in mind. Michael Rosenberg's answer is certainly high quality, and would most likely do well in competition with the answers to the earlier version. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 6, 2018 at 17:24
  • $\begingroup$ For the record: I don't claim to be innocent myself. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 7, 2018 at 4:08
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    $\begingroup$ This is just a personal attitude but I don't do anything on the site with a view to manipulating a certain class of users' rep. I just upvote helpful questions and answers and mark duplicates as duplicates. A duplicate can be helpful and so can its answers. But I can understand a former mod or a longstanding site contributor becoming fed up with certain behaviours. One shortcoming of deletion is the case when one answer generalises another, which are often marked as duplicates. Different users will often seek different degrees of generalisation at different times. $\endgroup$
    – Robert Frost
    Commented Oct 7, 2018 at 8:20
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    $\begingroup$ @RobertFrost I am not enjoying such deletions and downvotes either. But I am convinced that the practice of rampant addition of dupes is bad for the site. I don't really have a choice. The alternative is to twiddle my thumbs and watch the site get destroyed simply because the built-in incentives work towards increased entropy. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 7, 2018 at 14:40
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    $\begingroup$ The need to stop the site from turning into a homework mill trumps all other concerns. $\endgroup$
    – Jyrki Lahtonen
    Commented Oct 7, 2018 at 14:59
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    $\begingroup$ Changed my downvote to an upvote. My views have evolved to be similar to much of what you propose, except that I don't think we should delete dupes if any answer contains significant novelty. So I only vote to delete dupes when I am reasonably sure that the answers are all dupes too. $\endgroup$
    – Bill Dubuque
    Commented Feb 19, 2019 at 0:58

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