What should we do when one person tries to delete every duplicate?

I agree that questions that are duplicates should be closed. But, I find a disturbing pattern when somebody closes a dupe with a more general dupe and even not the correct tag i.e. suppose a person asks a question primarily in R tag with a regex on it, an the person who closes the question on a regex only tag, it may not be that helpful. I find multiple posts that I answered getting dupe tagged and then deleted with this way of closing. More disturbing attitude is when I find a similar question answered by the same person (may be next day or couple of days later). His prior actions suggest that he considers those questions not to be worthy for SO. When I dupe tagged the question, immediately he reopened it. It explains that the OP's concern is not really about cleaning the SO from those kind of questions. I am sure that moderators would take notice of that.

NOTE: I could provide many links to that, but then the identity of the person who does this will be revealed and I don't want to do that.

Update

After 8 months (Oct 15 2021)

1. Is there any changes in the behavior?

   None

2. Is the group still involved in deleting questions when others answer and then reopen when they themselves answer?

   Very much. I was surprised to see a dupe question getting reopened by the same group. They act as a group and there are evidence or here or here

3. Is it okay to answer in regex tag without their approval?

   No. Because I find in many occasions even in the last couple of days, the same guys closing questions that are not remotely dupes as dupes while gleefully answering dupe questions.

Not really an answer to this question, but something I think may be worth looking into:

What bothers me a lot more about these examples is that a significant portion of the deleted questions all have the same 2nd user casting a delete vote.
That can't be a coincidence, and has got to be coordinated delete-voting.

Frankly, I find that more concerning than a single user delete-voting duplicates he sees over and over again.

I often cast close and delete votes on regex questions (and answers) to keep the regex SO tag part clean, avoid having to re-type the same solutions time and again (sometimes several times a day) and easy to search for the regex solutions online without the need to go through several dupes before coming across the original solution.

Personally, I believe deleting every duplicated post is bad, and I never do that. Let me try to clear up my "moderating system" that is based on a single principle: I am only doing what others have been doing, certainly paying more attention to what others have been doing to my posts, since it is just more visible to me. I have been collecting the SO behavior patterns since 2015 when I started my active SO life (my profile says "Visited 2231 days, 2222 consecutive").

What posts do I close?

• Regex explanation requests and any post where OP is confused with truly basic regex patterns (for the majority of them, there are even specific signposts dealing with that exact issue)
• All questions with no shown attempt, with no efforts shown, where the answer is "use feature_name" (and that are usually answered in a "here-you-are" style) with a generic duplicate close reason (these include word boundary usage of several kinds) that is enough to solve the issue, usually leaving a comment with tips on how to proceed with asking. I agree I do not give the exact solution here, but the linked threads are fair source of knowledge OPs need to answer their questions March 13, 2021 update: as this behavior can lead to suspension, I have fallen back to the practice described in Should “Give me a regex that does X” questions be closed? and I vote to close an "Needs More Focus"
• Questions about some very common, extremely frequent regex topic, where OP might even share some basic efforts, but the real solution is in most cases already present or there are posts that can be easily adapted to OP needs (mostly questions about phone number, number, email validation and suchlike), though some are still valid on-topic questions
• Any post that has canonical dupe reasons (using variables in JavaScript, etc. regex, doubling backslashes inside string literals, using full string matching vs. partial string matching, etc.)
• Certainly, exact duplicates (that I can find within a single Web search and two result pages).

What kind of closed duplicate posts are worth downvote?

Not all of them deserve downvotes. I used to downvote posts I closed automatically some time ago, but I stopped because I realized users should have a chance to show they are eager learners. However, I do downvote when

• The duplicate is too frequent, and thus, "evident" (a basic search would have solved the issue and there would be no need to write an SO question)
• The duplicate was basically about a typo, or "bordering on" a typo (again, OP could have solved it with a tiny bit of extra attention)
• OP does not even try the solutions provided in comments (I often post the solutions), or/and behaves in an "emotional" way

Deleting a post is necessary when

• The post is misleading: when the answer is not actually the right solution even if it is accepted, almost all typo questions when there is no real problem, etc.
• The post is a duplicate of a very frequent and evident regex issue (some dupe reasons contain a hundred and more linked duplicates, and there is no risk losing value for SO when removing them), including number, phone, email, URL validation questions and the like
• The post owner did not put effort into solving the issue, or if that effort was truly insuffucient and was meant to disguise a pure code request.

When not to delete and even upvote duplicates?

• I never delete highly upvoted duplicates with long history, and even cast reopen votes in case I come across them
• When the question is related to a non-evident issue and the question is asked in a nice way, with code examples, test cases and explanation of OP efforts
• When the question is actually a duplicate of two SO threads (usually, they are still about very frequent issues, but the fact two links are required makes the current question a nice-have)

Note I use the terms "evident" to describe a simple and frequent issue with broad SO coverage, and do not mean that something should have been clear to OP prior to asking a question.

Now, it must be easy to understand the reason behind those closures/deletes listed in the question. This is not done to make anyone feel frustrated. If you think the duplicate question is unrelated, I assure you there was a reason. If you disagree, let me (or a person who closed the question) know via a comment, and if you have a point, the post will be reopened. If an answer is unique and is valuable, it is great, but what would you say to the identical XX valuable answers? Removing one or two (no, not all the threads I close and cast delete vote to get actually removed) does not harm anyone and SO either. In my opinion, this is in line with the official help page you quoted.

By the way, it is not true that a close reason with few votes is not a good close reason: some answers with just one upvote are worth 100 votes, they are simply not visible to others due to various reasons (poor title, or vice versa, too good title that people usually do not even think of to search for).

I disagree that "power" users should not use gold badge to moderate the corresponding tags: that is the purpose of gold badges. More, there is no problem with hammer-closing: any three users with reopen votes will reopen the question. Believe me, if the question was closed by mistake, it happens within minutes.

Our colleagues who frequent the 10K tools page do a great job keeping this place clean. Again, they are just human beings who can make mistakes. We all do. So, there is an undelete tool there, too. Don't you think it can also be perceived as another "sink to use up their undelete votes"? In my opinion, this system works in the end. If there are any disputes, I rely on moderators' final say.

I agree "it is not a good thing there is no real oversight on what gets deleted". Still, there is a list of recently deleted and undeleted posts at least.

I am not sure what "deletion behavior" that's been "going on for many years" you mention, my deletion behavior has been the same all the time I obtained the privilege. It is also easy to "copy" my close/deletion pattern if you

1. want to keep my preferred tag as clean as possible
2. try to keep an eye on as many posts in this tag as possible (sometimes, just browsing a question after question).

I want to also note that I also care to delete the questions I answered that turn out evident duplicates, see this answer, or this post, for example.

Now, after I wrote all this, I come to think I am overthinking it a bit, and probably mislead others with my decisions. I admit I have been obsessed with "aggressive cleaning" approach, but that was done in good faith. If this thread clears up best practices that would be real added value to the community.

You know what, I have bigger problems with the other part, saying that two questions are duplicates when they aren't or are just related questions. For the deletion of duplicates... I really don't find most deletions as irremediably harmful, for a simple reason: even if this user tries to do that, it will be fighting against every asker of Stack Overflow, which is a tall order even with some coordination.

I'm not against deletion of most duplicates, as there's a practical limit of ways to ask useful signpost questions without running into irrelevant keywords due to humans failing to accurately describe what they are asking about. I prefer people finding relevant information due to using the correct set of keywords than a crapshoot where you find questions where someone described something that happened to use the same keywords, but in a different context.

For me, the first issue is far more important and preponderant than the latter, as in the latter would be a lost battle of attrition against the internet, which would sort itself out (aka without intervention of anyone). The other doesn't have this characteristic and as it's very difficult to convince others that two questions are actually not duplicates of one another.

_{Disclaimer: I'd rather not discuss the specific user, or their deletions. This answer's focus is on the rationale behind why a user may vote to delete questions after marking as a duplicate and what actions another user could do if they notice this behaviour.}

I have to admit that I often also vote to delete a question after marking it as a duplicate.

My reason is that I never spend longer than 5 minutes searching for a duplicate and if I can find a duplicate within those 5 minutes then it really doesn't need another signpost. Instead, what often happens is users find a way to still post an answer to the already closed question and thus posting either rehashed information or posting an answer that should be added to the duplicate target.

What to do when one person tries to delete every duplicate?

If you really want to, flag a post for moderator attention, leave a note explaining why it's flagged and then leave it at that so the moderator(s) can decide if anything further needs to be done.