What do I do?
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Since I'm the one who more-or-less kicked off this discussion, I thought I'd weigh in on how I decide whether two questions are dupes.

Mandatory disclaimer: I vote according to my own conscience, based partly on how I've seen other high-rep users on this site vote, partly from my own understanding of this site's policies and practices, and partly on my own (evolving) understanding of what it means for a question to be a dupe. Despite being a [pretty high-rep user][1], I'm still fairly new to the community (less than a year), so I might be wrong - I'm often wrong, in fact - but this is the measuring stick I use.

So let's say I have questions A and B. In my mind, B is a dupe of A if:

 1. B is asking the same question as A. This is pretty uncontroversial; http://scifi.stackexchange.com/questions/40862/when-does-the-doctor-actually-tell-river-his-name and http://scifi.stackexchange.com/questions/47097/how-does-river-song-know-the-doctors-name are pretty transparently asking the same question (there may be better examples of this, but this was a recent one that was easy to find).

 2. B is asking a different question to A, but the answer to A is the answer to B. This is a little controversial, but I don't think it's terribly so. Let's look at some examples:
  - http://scifi.stackexchange.com/questions/87680/did-bellatrix-really-love-her-husband and http://scifi.stackexchange.com/questions/63427/did-bellatrix-have-any-affection-towards-voldemort are slightly different questions. The first is about Bellatrix's relationship to Rodolphus, the second is about her relationship to Voldemort. However, these two questions have basically the same answer; alexwlchan and I use the same quote, but frame it in slightly different context.

     However, the framing context has no bearing on whether or not the answers are the same. The existence of my answer does not benefit the site. It also doesn't actively harm it, which is part of the reason I'm not freaking out trying to get it deleted<sup>1</sup>

 3. **Any** answer to B would also be an answer to A. This is probably the most controversial, since it doesn't rely on existing answers, and I suspect I'm in a minority for feeling this way. I know that I've done this before, but I can't think of any other examples off the top of my head. 

 However, the reason I vote this way is because (and I know there's a meta discussion on this, but can I find it? Nooo.) the fact that A doesn't have useful answers doesn't affect whether or not B is a dupe. That's actually part of the reason I VTC'd http://scifi.stackexchange.com/questions/91922/what-is-tolkien-trying-to-say-in-this-letter. Wad, the OP on that question, pointed out that the answers to the dupe (http://scifi.stackexchange.com/questions/28876/was-elrond-in-tolkiens-opinion-more-inherently-powerful-than-galadriel) weren't satisfying because they were based on opinion rather than from authorial intent. This is completely true.
 
      However, the top two upvoted answers (and arguably the third as well) would both be perfectly valid answers to Wad's question. Would they be satisfying? No. But they'd be upvote-worthy, because they would be an earnest attempt to answer the question.

      Equally true, suppose we found a way to get an answer to Wad's question based on authorial intent. That answer would *also* be a correct answer to the dupe question. It would in fact be more correct than the other answers to that question, for the *same* reason it would be an answer to Wad's question: we actually find out what the author thought.


Why do I do it?
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What I've found to be true about these answer-dupe questions is that, when you essentialize them, they are the *same* question; Izkata in comments relates this to the notion of a "parent" question, and you can apply this rule to every example I've given:

 - The *Doctor Who* examples in my first point are both different ways of asking "How does River know the Doctor's name?"<sup>2</sup>

 - The Bellatrix questions in my second point are just different ways of asking "Who does Bellatrix have feelings for?" (essentialization courtesy of Izkata, who got be thinking about this)

 - The Tolkien questions in my third point, the questions that sparked this discussion, are both different ways of asking "Did Tolkien think that Elrond was more powerful than Galadriel?"

What we're doing by closing these questions as answer-dupes is not fundamentally different from what other SE sites suggest doing: [canonical questions (or answers)][2]. The difference in our approach is twofold:

 1. We let *every* question potentially be a canonical question
 2. We don't wait for a question to become a "problem" question

What are the benefits?
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phantom42 already laid them out. Go read his answer.

What are the downsides?
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This list is largely going to be based on comments I've seen other users making in these discussions, and my responses to them. But I'll try to throw in some originality too

 - **If question A and B are different, the part of A's answer (unrelated to question A) that addresses B may be incorrect**. This is definitely a potential problem, but it's a problem related to *all* answers. No information in any answer, no matter how tangential or unrelated to the question its been posted under, should be immune from the community vetting process; incorrect information should be *corrected*.

 For example, consider my answer to [Is MODOK planned to appear in the MCU?][3] (I've linked to a specific revision to make my point better). I initially wrote this answer as:

 >In the game, [MODOK is] an enhanced version of Aldrich Killian, the main antagonist from *Iron Man 2* and founder and CEO of Advanced Idea Mechanics (A.I.M.).

 I wrote that line from memory, which was incorrect. If somebody else had asked "Who was the main antagonist of *Iron Man 2*?", and if that question was closed as a dupe of this MODOK question, **and** my answer was left as it was, *that would be a problem.* Without that hypothetical question, my answer being left as it was *would still be a problem*.

 Of course what happened was that I had to leave for a family engagement, and I was unable to correct the information. Someone commented (I don't know who it was; I only know the comment was made because I received a notification on the mobile app, which I part of, but the comment was deleted before I could address it)  that this was incorrect, and I was unable to redress it. So Keen, a subject-matter expert on the Marvel Cinematic Universe, corrected it for me.

 That's an example of the system working properly. *Another* example of the system working properly is on http://scifi.stackexchange.com/a/92282/31051, and an answer which has since been deleted. The answer read:

 >I would just add that while the movie makes Fili and Kili die at the Battle of the Five Armies, they in fact die in the Moria (as per JRR Tolkien), during the failed attempt from Balin to reclaim it from the Orc.

 >The last pages of the diary of the colony of dwarves of Moria have been written by Kili, which has a writing elfish in nature (perhaps why he has been chosen by Peter Jackson to be the subject of the invented Love Story with Tauriel...).

 This information is obviously incorrect, and it *so* incorrect that the answer is beyond salvaging; it could not be edited without substantially deviating from the answer's intentions. The correct course of action is to downvote the answer, and eventually delete it from the site altogether, which is exactly what happened; the answer attracted 13 downvotes and no upvotes, and was deleted by its owner 14 hours after it was posted.

 Other users have commented in this discussion that sometimes incorrect information gets called out in comments, and then never corrected. This is absolutely a problem, but it's a problem with our policy on editing posts, not on our dupe policy. If this is a problem that we're currently grappling with (and it might be; I've not seen it myself but what does that matter?), then we should be dealing with it on *all* answers.

 - **Questions might be closed as dupes based on a throwaway one-liner in an answer to an unrelated question.** My other comments in this discussion notwithstanding<sup>3</sup>, I actually agree with this one. Questions should *not* be closed-as-dupe based on insignificant, tangential parts of answers to other questions; and you'll notice that none of my points above involve this.

 Picking on myself again, consider my answer to http://scifi.stackexchange.com/questions/80061/how-could-treebeard-call-for-war-without-an-entmoot/80063#80063. In that answer, I have a footnote which reads:

 >[Quickbeam, an Ent is] so named because he once replied "Yes" before another Ent had finished asking the question.

 Imagine if someone asked "Why is Bregalad named Quickbeam?" That would be a terrible question because it's answered immediately following the character's introduction, but imagine. If that question were then closed as a dupe of http://scifi.stackexchange.com/questions/80061/how-could-treebeard-call-for-war-without-an-entmoot based on my answer, I would have a problem with that; if I took that single line, in isolation, and made it an answer, I would be disappointed in myself. That line is *not* an answer.

 However strongly I may agree with our current policy of closing questions based on answers, I do agree with this point; we should apply the same scrutiny to those answers as we would to answers on the question we're closing.

 - **Closing based on answers means that the user who asked the closed question has to go hunting for the answer to their question.** This is true, but I think it's more a limitation of *how* we're applying this policy, not of the policy itself. Obviously this situation would be improved if [the "this question is a dupe" dialogue pointed to the particular answer][4], but that's not something that's going to happen. One thing I often try to do is leave a comment saying "This is a dupe of question X, and in particular look at user Y's answer", which may help the questioner but may not help future travellers (since comments are ephemeral and subject to deletion at any time).

 Ultimately this isn't a bad point, but I feel like it's directed at the wrong policy. We *already* have systems in place that encourage questioners to seek out and read related questions to find the answer, rather than asking a dupe question: the existence of the duplicate close function tends to act like this, and can be perceived by new users (and old ones) as a slap on the wrist; we downvote posts that show a lack of research effort, although that's thankfully limited to questions that are answered by a simple Google search; we have "similar questions" boxes that pop up when you're creating a new question, showing you possible examples of similar questions that have already been asked (these boxes are generally pretty awful, but that's a different issue).

 If the objection to the current policy is that it makes users look harder for the answer to their question, then I think we should be having a broader conversation about whether we should *ever* be closing questions as duplicates, because that seems like an unavoidable result of the system in general.

 - **The answer to the old question hasn't been properly vetted as an answer to the new question.** This objection hinges primarily on the fact that closing a question only requires five votes from 3k+ users, but up- or downvoting an answer is open to the entire community (who have at least 15 rep for upvoting or 125 rep for downvoting).

 I'll leave aside the fact that many correct answers get fewer than 5 upvotes ([there are currently 4100 accepted answers with scores of 5 or lower][5], accounting for 30% of all accepted answers, although that excludes correct answers that aren't accepted).

 As with the last objection I discussed, this is something that is true in a broader context. It only takes five votes (or one modhammer, or one gold tag badge for dupe votes) to close a question for any reason, and to argue that those five people may not be correct is an argument better levelled at the entire closing policy.

 It is absolutely possible for close voters to get it wrong, on any kind of close vote. That happens, because we're human and make mistakes, and because we have different ideas about what each close reason means. But there is recourse for users who disagree with this, of *all* rep levels:

   - Any user can comment on their own posts, and users with 50 or more rep can comment on any. It's somewhat regrettable that comments don't bump the question (although on balance it's probably better that they don't), because that does limit the exposure of these questions, but recent questions that are still near the top of the list are going to get viewed by plenty of users who can cast votes anyway

   - You only need 15 rep to flag a post for moderator intervention. The modhammer can open a closed question with only a single vote.

   - 250+ users (of which there are about 2000) nominate their own question for reopening, and 3k+ users (of which there are currently over 200) can cast reopen votes on any question. It may take five votes to close a question, but it also only takes five votes to reopen it

   - [You only need 5 rep to participate in Meta][6] at which point you can bring your case to that subset of the community (mostly long-time, experienced users) who participate here.

 And bear in mind as well that a new question is going to get seen by plenty of users who are able to flag, comment, or vote on it to get it reopened, and casting reopen votes sends it into the queue where it can potentially be seen by *any* 3k+ user (with a time delay so you don't get the same users who VTC'd it immediately voting to leave it closed).

 If after all of this the question remains closed, unless most 3k+ users just don't care about voting (which is a much larger problem), I submit to you that the expert community *has* vetted the duplicate answer.

 - **If the dupe question doesn't have satisfying answers, the asker of the closed question doesn't have many options for getting them.** This is absolutely true, and is a side-effect of duplicate closing in general. On more than one occasion I've flagged questions as dupes of other questions that have *no* upvoted answers, because the questions *are* dupes.

 I agree that this is a problem, which I don't have a good solution to (the typical answer is "post a bounty", but you need 75 rep to do that, at which point the minimum bounty would basically wipe out your site privileges; it's not something that works for all users). I'd be open to discussing ways to solve this problem, but as it stands I feel (and the community both on SFF ans the broader SE network seem to agree) that the benefits of a duplication policy outweigh this problem

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<sup>1</sup> Sidenote: Yes, my answer probably *should* be deleted. I jumped the gun, answering before I'd checked to see if it was a dupe, and only found the dupe after my answer had been accepted and could no longer be deleted. *C'est la vie*.

<sup>2</sup> Of course that's the actual title of one of the questions, which is convenient for us; but I maintain that it's not strictly necessary

<sup>3</sup> I got a little upset, and said some things I didn't mean. I'm better now

  [1]:http://stackexchange.com/leagues/82/alltime/scifi/2011-01-11/31051?sort=reputationchange#31051
  [2]:http://blog.stackoverflow.com/2011/01/the-wikipedia-of-long-tail-programming-questions/
  [3]:http://scifi.stackexchange.com/revisions/90036/1
  [4]:http://meta.scifi.stackexchange.com/questions/4708/when-i-flag-as-dupe-can-the-system-put-the-link-to-the-answer-and-not-the-quest
  [5]:http://scifi.stackexchange.com/search?q=is%3Aanswer+isaccepted%3Ayes+score%3A..5
  [6]:http://scifi.stackexchange.com/help/privileges/participate-in-meta