1. They overwhelmingly are not moderators, and they are not doing moderation by closing questions. This is curation, and duplicate closures overwhelmingly are done by subject-matter experts: users with a gold badge in one or more of the tags originally applied to the question. The requirement for such a badge is based on answering questions:
> Earn at least 1000 total score for at least 200 non-community wiki answers in the $TAG tag. These users can single-handedly mark $TAG questions as duplicates and reopen them as needed.
So these are definitely not people averse to the idea of answering questions.
2. I can guarantee you that the overwhelming majority of these cases are not people trying to be "mean". Users are actively incentivized against closing duplicates, which has historically led to nowhere near enough duplicate questions being recognized and closed (although there have been many proposals to fix this). Dupe-hammering questions "to be mean" is considered abusive, and suspicion of it is grounds to go to the meta site and discuss the matter.
No, people close these questions because they genuinely believe the question is a duplicate, and genuinely believe they improve the site with this closure. It's important to understand that: a) people who ask a question are not entitled to a personalized answer; b) leaving duplicate questions open actively harms the site by allowing answers to get spread around, making it harder for the next person to find all the good ones; c) the Stack Overflow conception of duplication is not based on just what the OP understands or finds useful, but on what everyone else afterward will find useful.
For example, there are over a thousand duplicate links to https://stackoverflow.com/questions/45621722 , most of which is from my own effort — spending many consecutive days closing dozens of questions a day (and/or redirecting duplicate closures so that everything could point at a "canonical"). Yes, that's a question about how to indent Python code properly. I identified candidates for this from a search query and carefully reviewed each one, verifying the issue and sending other duplicates to more specific canonicals in many cases (such as https://stackoverflow.com/questions/10239668). And put considerable effort into improvements to questions and existing answers, writing my own answer, and adding links and guidelines for other curators so that they can choose more appropriate duplicate targets in some cases. I also looked at a wider search that probably had a fairly high false positive rate, but implies that there could be thousands more that I missed.
3. When your question is closed as a duplicate, you immediately get a link to an answer. You don't even need to wait for someone to write it! It's someone saying "here, I was able to find it for you, thanks perhaps to my familiarity with other people asking it".
4. Stack Overflow users really do "try to find what's different about" the question. It just... doesn't actually matter in a large majority of cases. "I need to do X with a tuple, not a list" — well, you do it the same way. "I need to Y the Xs" — well, it seems like you understand how to Y an X and the real problem is with finding the Xs; here's the existing Q&A about finding Xs; you shouldn't need someone else to explain how to feed that into your Y-the-things loop, or if you do, we can probably find a separate duplicate for that. Things like that happen constantly.
Sometimes a question shows up with multiple duplicates. This almost always falls into two patterns: the user is really asking multiple separate things (due to failing to try to break up a problem into logical steps) and each one is a duplicate; or the question is constantly asked but nobody knows a good version of the question, and gives multiple links to previous attempts out of frustration with constantly seeing it. (The latter is bad; someone is supposed to write the good version and send everything else there. But that typically requires behind-the-scenes coordination. Better would be if the first bad attempt got fixed, but you know.)
5. Closing a question is emphatically not about telling people to go away. The intended message (unless the question is off topic or the OP just made a typo or had a brainfart) is "please stay and fix this". However, it's perfectly reasonable that an explicit attempt to catalog and organize useful information treats redundant indices by pointing them at the same target rather than copies of the target. And questions are indices in the Q&A model.
> Earn at least 1000 total score for at least 200 non-community wiki answers in the $TAG tag. These users can single-handedly mark $TAG questions as duplicates and reopen them as needed.
So these are definitely not people averse to the idea of answering questions.
2. I can guarantee you that the overwhelming majority of these cases are not people trying to be "mean". Users are actively incentivized against closing duplicates, which has historically led to nowhere near enough duplicate questions being recognized and closed (although there have been many proposals to fix this). Dupe-hammering questions "to be mean" is considered abusive, and suspicion of it is grounds to go to the meta site and discuss the matter.
No, people close these questions because they genuinely believe the question is a duplicate, and genuinely believe they improve the site with this closure. It's important to understand that: a) people who ask a question are not entitled to a personalized answer; b) leaving duplicate questions open actively harms the site by allowing answers to get spread around, making it harder for the next person to find all the good ones; c) the Stack Overflow conception of duplication is not based on just what the OP understands or finds useful, but on what everyone else afterward will find useful.
For example, there are over a thousand duplicate links to https://stackoverflow.com/questions/45621722 , most of which is from my own effort — spending many consecutive days closing dozens of questions a day (and/or redirecting duplicate closures so that everything could point at a "canonical"). Yes, that's a question about how to indent Python code properly. I identified candidates for this from a search query and carefully reviewed each one, verifying the issue and sending other duplicates to more specific canonicals in many cases (such as https://stackoverflow.com/questions/10239668). And put considerable effort into improvements to questions and existing answers, writing my own answer, and adding links and guidelines for other curators so that they can choose more appropriate duplicate targets in some cases. I also looked at a wider search that probably had a fairly high false positive rate, but implies that there could be thousands more that I missed.
3. When your question is closed as a duplicate, you immediately get a link to an answer. You don't even need to wait for someone to write it! It's someone saying "here, I was able to find it for you, thanks perhaps to my familiarity with other people asking it".
4. Stack Overflow users really do "try to find what's different about" the question. It just... doesn't actually matter in a large majority of cases. "I need to do X with a tuple, not a list" — well, you do it the same way. "I need to Y the Xs" — well, it seems like you understand how to Y an X and the real problem is with finding the Xs; here's the existing Q&A about finding Xs; you shouldn't need someone else to explain how to feed that into your Y-the-things loop, or if you do, we can probably find a separate duplicate for that. Things like that happen constantly.
Sometimes a question shows up with multiple duplicates. This almost always falls into two patterns: the user is really asking multiple separate things (due to failing to try to break up a problem into logical steps) and each one is a duplicate; or the question is constantly asked but nobody knows a good version of the question, and gives multiple links to previous attempts out of frustration with constantly seeing it. (The latter is bad; someone is supposed to write the good version and send everything else there. But that typically requires behind-the-scenes coordination. Better would be if the first bad attempt got fixed, but you know.)
5. Closing a question is emphatically not about telling people to go away. The intended message (unless the question is off topic or the OP just made a typo or had a brainfart) is "please stay and fix this". However, it's perfectly reasonable that an explicit attempt to catalog and organize useful information treats redundant indices by pointing them at the same target rather than copies of the target. And questions are indices in the Q&A model.