I'm going to leave my original answer, which was partially in response to Frostfyre's query about the closure of his question and partially an introduction to the issues of judging duplicates. This answer is focuses solely on the issue of judging duplicates.
There might be a better word out there, but my head's chasing all the ideas for this answer and can't think of it. "Duplicity" usually identifies something deceitful or fraudulent. A person with a duplicitous nature identifies a person who, for example, has ulterior motives.
However, I'm going to wantonly use "duplicity" to mean "the state of being a duplicate." If you can think of a better word, lemme know.
Why judge at all? Why not just answer the @#&! question?
Worldbuilding.SE is the most creative, imaginative, and therefore the most subjective stack in the Stack Exchange galaxy. That's actually a problem, because Stack Exchange designed its service to accommodate very objective, clear, precise, concise, specific, even obvious questions. In the world of programming, for example, there may be a dozen different ways to approach programming for a specific effect or need — but the effect or need is very specific and it's obvious that anyone asking about that specific effect or need is asking a duplicate question.
Judging duplicates serves two important needs on all stacks.
It brings the many different ways of looking at the same problem into focus. This brings value to the overall stack by gathering all the best answers into one place with the several (sometimes many) duplicates all pointing to that one source. Even knowing that your perspective is a duplicate of other perspectives helps
programmers stack users to see better how to solve their problem.
It reduces clutter on the stacks. If you don't mark duplicate questions as duplicates, (a) you get the same answers repeated over and over and over and over and over and (b) good answers are not all in the same place, meaning greater research is needed to find them, meaning the quality and usefulness of the stack is reduced.
A substantial component of how we do things here at worldbuilding.SE is based on Stack Exchange's expectation of how their service (not our service) is to be used.
Unfortunately, we deal with stories. I've been a micropublisher, and I can honestly tell you there are only the proverbial nine unique stories. Period. Boy meets girl, battles robot, saves the girl. I can't tell you how many tens of thousands of times (I mean that, we processed 3,000-5,000 manuscript submissions each year — as a micropublisher) an author would tell us "there's nothing like my story out there!" or "This is a completely new and unique story!" No, it isn't. There's no such thing.
But there are an infinite number of ways to tell those stories. And that's where publishers look for uniqueness. The flavor, the ambience, the worldview....
And that's the problem we have with judging duplicates. Where 99% of the users of (e.g.) Stack Overflow will likely agree that two questions are duplicates of one another, maybe only 70% will here (or less) because stories are far more interpretive than, e.g., programming — and the answers of question #1 might not actually address the specific problem of question #2. Or maybe they do and the OP of question #2 is just being stubborn and short-sighted. Or maybe they don't because it's the judges who are being stubborn and short-sighted. See the problem?
You stupid engineers...
Finally, before actually getting to the meat of this answer — I'm an engineer. I've also been a publisher and am a writer (both technical and artistic). You might not believe it, but this is a useful perspective. I've had to deal with authors who don't see the difference between artists and engineers. If you'll forgive the simplification:
Artists live in a world full of rules, but don't want to believe it and often pay little or no regard to those rules. They're trained that way. They need to see the whimsy of life and expression of imagination to be good at what they do. Consequently, they often have difficulty being published because publishers are businesses (rule-driven hierarchical organizations) that must pay their employees.
On the flip side, all engineers see is rules. We're trained that way. We literally can't do our jobs without both understanding the rules and regularly using processes that create new rules. We can't afford to give up those rules or bridges collapse and people die. We also have difficulty being published because consumers are emotional and are looking for an escape from their rule-burdened lives.
It's the people who can merge the artist and the engineer within who are often most successful.
Regrettably, what you're about to experience is 98% the engineer. Fair warning. Let's begin.
Titles, Questions & Answers
I suspect a common habit is to look at the titles of the two posts and judge their duplicity based solely on those few words. I was guilty of that early in my experience with WB.SE. Unfortunately, while a valid point of judgment, titles are also the weakest aspect of two posts to compare.
A thorough reading of the body of both posts (the question) is mandatory for a comparison. Someone who skims over the two without really considering the many aspects of a question (conditions & restrictions, background or premise, and the question itself) is providing a superficial judgement.
But I wholly agree with TimB II that the answers must also be taken into consideration. But this needs a grain of salt because a post may not have been answered sufficiently (even though it was answered adequately) which affects the judgement.
So, to begin, good judgement requires evaluating the titles of the two questions, the body of the two questions, and the answers to the first question. If you're not willing to take the time to do that, please consider not voting.
2+2 = 1+3 = 1.8+2.2 = 421-317... or do they?
I'd like to introduce my evolving set of rules for judging duplicates. This is, of course, my own opinion. Don't let my use of the word "axiom" fool you.
Axiom 1: If two questions can only be answered with exactly the same answers, then they are duplicates.
This first axiom is completely true and, after considering what Mithrandir24601 had to say, almost completely useless. Not completely, but darn close. Unlike our more objective sister-stacks, questions asked here are rarely perfect duplicates, which underscores the need to evaluate answers along with the questions when judging duplicity.
Corollary 1.1: If two answers can only be the result of exactly the same questions, then the questions must be duplicates.
This corollary is almost as useless as its axiom in that the world of writing is almost never this neat and tidy. But I've found it useful because comparing how respondents have interpreted questions has proven valuable to my judging process. And this deserves a quick tangent.
OP Intent: The original poster of any one question posted that question with a specific, if not well defined, intent or expectation.
Respondent Interpretation: Respondents have no choice but to interpret the question to ascertain the OP's intent because there's no such thing on Worldbuilding.SE as a "perfect question." There will always be a disparity between the OP's intent and the respondent's interpretation — especially among our more creative respondents (bless them all!) who will throw a curve ball at the OP just to stir things up.1
Axiom 2: If there is a variance between the OP's intent and the respondents' interpretations, in keeping with Stack Exchange's policy of preserving OP intent (...to clarify the meaning of a post without changing that meaning...), it is the OP's intent that must be preserved and judged when considering answers as a basis for duplicity.
Case: more vs. less
Axiom 3: If the two questions are asking about the same thing and differ only in conditions, limitations, or restrictions, then they might be duplicates.
Corollary 3.1: If the conditions, limitations, or restrictions of Question #2 narrows the question as found in Question #1, then Question #2 is a duplicate of Question #1 because the answer to Question #2 could be found associated with Question #1.
Corollary 3.2: If the conditions, limitations, or restrictions of Question #2 expand the question as found in Question #1, then Question #2 is NOT a duplicate of Question #1 because there can be valid answers to Question #2 that cannot be found associated with Question #1.
That was ugly to read, but what it boils down to is that narrow questions are always duplicates of their broad counterparts. Sometimes...
Corollary 3.3: If the conditions, limitations, or restrictions of Question #2 narrow the question as found in Question #1 but demonstrate a critical understanding or unique perspective unlikely to be casually found in an answer to Question #1, then Question #2 is NOT a duplicate.
And there's that fuzzy border. Without the OP of Question #2 making a darn good case for uniqueness, identifying that border is very subjective. Remember, the people who use this site range from untrained children to highly trained adults that don't have the training other highly trained adults have. In other words, where one person may see a very clear distinction that makes this an easy judgement for them, another may not and yet feel like they have a clear and justifiable perspective. Ugh.
Lemma 3.3.1: While it's true that Stack Exchange does not require anyone to justify any vote or flag, considering how subjective the subject matter on this site is, you should err on the side of leaving an explanation in comments — if only so people can contact you for clarification.
Case: Near misses
Axiom 4: If Question #2 is similar to Question #1 in ways that cannot be attributed to conditions, limitations, or restrictions then they might be duplicates.
Corollary 4.1: If from the perspective of someone skilled in the art or science necessary to answer the question the two questions are substantially the same, then they are duplicates.
Corollary 4.2: If If you are NOT skilled in the art or science necessary to answer the question, you should err on the side of posting a comment asking for clarification before voting to close the question as a duplicate.
If you thought the last axiom and its corollaries were fuzzy, this one is really fuzzy. It's basically asking you, the judge, to put yourself into the shoes of people who know more than you do. Said another way, it's demanding that you, the judge, accept the fact that you don't know everything. That's a lot easier said than done! But it must be done. If the comparison requires knowledge you, the judge, have not been trained to, then you really should proceed with caution. Yes, it's just a website and just a hobby (at least it should be just a hobby!), but you'd be surprised how upset people can get over hobbies. After all, what you're doing is another form of calling someone's baby ugly. Since the OP of Question #2 will at some level take it personally that their question was closed, please be sure you're right.
Case: When all else fails...
Axiom 5: Protecting the integrity and intent of Stack Exchange should not take precedence over helping an OP solve their problem.
I have no doubt that there's a lot of people who use or have used this site who think I don't adhere to that axiom. They might be right. I'm not perfect. But I am an engineer, so it shouldn't surprise anyone that I default to following rules.
However, it's important to realize there's a balance between meeting Stack Exchange's expectations and the OP's needs. You can't simply ignore one in favor of the other. The OP needs and answer, but it isn't our site. Many people consider places like this (or Facebook, or Reddit, or any other web site) to be "public property" where the rules of free speech reign. In reality, all web sites are the equivalent of private property. Each and every one of us is a guest — a guest who's expected to follow the rules of the host.
As I explained earlier, there's good reasons why Stack Exchange wants questions marked as duplicates. Those reasons should be respected, and so we should have an eye open for the purpose of identifying duplicates when they happen. But we also need to be on the lookout for those gems, those questions that expose an amazing perspective that deserve to be left open. That can be hard to do because one of the most valuable aspects of Stack Exchange is its belief that the answers should help everyone, not just the person who posts the question. In the end, that might be a useful perspective, so let's add it as a corollary.
Corollary 5.1: If the answer to Question #2 is useful to no one other than the OP who asked Question #2, and Question #2 is a near-miss duplicate of Question #1, then Question #2 is a duplicate of Question #1.
OK, my question's a duplicate, but the answers to Q#1 don't help. What do I do?
I think we often forget that there are things we can do — and should do — to solve the OP's problem when said OP's question is marked a duplicate.
We can set a bounty on Question #1 and ask for more answers. We can link to Question #2 in the notes to that bounty so people see why the additional answers are sought. If a low-rep OP whose question has been closed feels this should happen, use comments and the
@user tag to contact one of the higher-rep users in the comment chain (or visit The Factory Floor chat room) and ask said person if they'd be willing to sponsor the bounty. We should all be willing to do that from time to time, just as lawyers are supposed to do pro bono work from time to time. Occasionally we need to be reminded of that. 😀
We can post a new answer to Question #1 ourselves. Indeed, maybe it should be a habit of worldbuilding.SE users to look a newly-closed-as-duplicate questions, hop back to the first question, and see if there's an answer we can post. If I remember correctly, there are even badges awarded to people who successfully answer old questions.
Judiciously,2 we can edit Question #1 to make the needs of Question #2 more clear in Question #1. This causes Question #1 to bounce to the top of the "Active" list so that it's visible again and can be more thoroughly answered.
1 If you want an example of this, go read the answer Willk gives to almost any question. He's a pro at throwing curve balls that open the OP's eyes to possibilities they might never have thought of. That's an incredibly valuable skill on this site. It also muddies the waters when judging duplicates something awful. I wish there were more users like him on the site.
2 I say "judiciously" because I'm not a fan of editing questions on this site. It's far too easy to change the OP's intent here. That's a consequence of this site being so naturally subjective. Therefore, while I promote editing Question #1 as a solution, I beg people to be cautious with their question edits. Please don't invalidate Question #1 or its answers in your effort to get answers useful to Question #2.