When a question is closed as an exact duplicate, the announcement always includes the statement "its answers may be merged with another identical question". How long after the closure does this occur? Who gets assigned to do it and under what policies does this person operate?

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Merging is an action that can be performed only by moderators, and this is irreversible (at least, not without getting hold of the developers to un-splice the questions). We merge duplicates very rarely, and when we do, only if it absolutely is a good thing to do. I'll explain why.

Theoretically, merging is useful for duplicates because:

  • Multiple answers to the same question can be collected in "one place" for convenience.

  • It allows a later, better answer to be attached to the older, possibly canonical question.

However, in practice, it has a lot of issues that outweigh its utility:

  • Question bodies are not merged. The "source" question has the question stub left behind, so its answers, when merged with the target, appear as solutions to the target question. Unless the two questions are very similar, the lack of background/context can confuse the reader.

    As a simple example, consider a question on the appearance of a General::ivar error. This can be due to a number of different functions (say, D and Series), but the underlying reason is the same. Suppose we had question A that had an MWE using D[...] and answers to it, and question B comes along with an MWE using Series[...] and also attracted answers. Now, merging the two would result in an answer to A that doesn't remotely resemble the example in the question.

    Given that we receive multiple answers for certain kinds of questions, half of which will be a "misfit" on the other question, merging questions will only make things messier and unorganized.

  • One of the reasons we keep duplicates around is because they aid in Google searches — the question keywords/phrases provide an alternate search route. When merging, the source question (the stub) is auto-deleted after 30 days, hence inaccessible to the spiders.

  • If both questions have accepted answers, the one from the source question will be unaccepted, depriving the answerer of 15 rep and the question asker of their question rep. We've had some folks complain in the past about lost rep from a merge.

  • As I mentioned earlier, there is no going back; at least not with the tools available to moderators. There are very few actions that we can take that are irreversible and this happens to be one of them.

So while there are questions that technically could be merged, it requires a lot of effort to first ensure that the match is pretty strong and that little to no mismatch exists between the two questions and answers (in terms of content, MWE, etc.). This certainly is time consuming and not worth the effort for little to no gain. Besides, the duplicate questions are cross-linked in the sidebar, so there is a trail from A to B and B to A.

At present, we merge only in cases such as when a question is cross-posted between Stack Overflow/Math and here, and their copy gets migrated (with answers) or if the OP repeated their question instead of editing/bountying it (typical for new users), etc. In other words, only when the two posts are more or less the same question by the same user and the double posting happend accidentally.

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I must add: we also discuss before merging questions (if needed) and I've nuked user accounts with far less hesitation than merging two questions. –  rm -rf Jan 4 '13 at 6:13
    
I see: when a question is closed as an exact duplicate, almost always in this context, "exact" doesn't mean exact enough to merge. –  m_goldberg Jan 4 '13 at 17:39
    
@m_goldberg That's more or less the case. If I could rephrase it, I'd probably write: "The underlying problem/concept behind the questions are the same and the answers to one are applicable to the other if replaced with the current context." This avoids the overly precise wording that exists at present. –  rm -rf Jan 4 '13 at 17:49
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