Should duplicates be downvoted as a punishment for not searching first, or is just closing enough?
I have several times asked a question that the "similar questions" search did not identify that I later found to be a duplicate of an existing question when someone pointed it out. I think, then, in the general case that a downvote is probably not appropriate because it is entirely possible that the user did search and did not find a question like their own in the list. The best solution is merely to close it with the reference to the existing question unless the question is bad in some other respect.
It's an easy mistake (posting duplicates). I confess to doing it myself more than once, despite having searched before. This is simply because each person uses a different vocabulary to describe the same thing. It happens.
To answer your question, I don't downvote for simple mistakes - I might if someone made a habit of it.
Downvotes express: the corresponding question lacks in quality.
Sure, the fact that a question is a duplicate can mean: lack in quality of doing prior research.
But depending on the subject and community, people might have done some research, and just didn't find the right duplicate. It is really not like the built-in search works that good, is it?!
Thus, it is fair to downvote a question that:
- obviously didn't see any prior research OR
- really lacks in quality in general
That is my criteria, and that results in plenty of "duplicate" questions not receiving a downvote, sometimes even an upvote.
One can also turn this around: "moderation work" doesn't stop with a downvote. When I vote down, I vote "close", too. And often "delete" on top of that. So a question that gets my downvote falls into the category "not good enough". And whether the question is a duplicate is really just one aspect of my considerations.
If I think the Internet will not benefit from the question being in the Google index I downvote both the question and all answers, this makes it more likely the question will automatically be deleted.
- if googling the title of the question find the duplicate with the first few papers of results
- if the title is unclear so will not help someone to know if the question is useful to them in the Google results page
- if the question is about an error, and googling the error text finds the duplicate
- If the duplicate is on the first page of the tab of frequent question for one of the tags the author used or should have know to use.
But I will upvote a duplicate question if it explains the problem in a way that is likely to result in other people finding it on Google, but not finding the original question.
At times I wish I could say
This question will not be useful to other people, but the author made a good effort (and it likely to be a net benefit) to the site, hence give them rep.
Closing a question as a duplicate is not a punishment.
The reason to ask a question is to get an answer, and if a question has been answered before, then closing as a duplicate is a way of saying "Hey, your answer is over there".
Otherwise standard criteria about question quality should apply, as with any other question.
There was an instance, in which I asked a question and the question didn't get any attention until someone else later asked the same question, and got answers to the question, in which case my question was closed as a duplicate of a newer question and downvoted. I think when deciding whether to downvote an old question because it is a duplicate you should check the date to make sure it is newer than the question it is a duplicate of as an asker cannot predict that someone else will later ask the same question. More general I think if you downvote an old question for lack of research you should check to make sure that any results you find are older than the question itself.
The only valid reason to downvote a dupe aside Jon B's answer would IMHO be to discourage answers, which may themselves be dupes of the duped question's answers or worse, turn out to be good answers the duped question itself did not have and will not be read by anyone not finding the dupe first.
However, downvotes for that reason should also be removed after dupeclosing because they only judged the fact of an already existing very similar question, but not the question's usefulness itself.
edit taking back that "dupe!"-downvote would be made easier if this feature-request was implemented.
Don't downvote a post on account of being a duplicate.
Ideally, a duplicate should be marked as such, and then either closed - with 0 votes, 0 comments, 0 answers (except regarding its dupeness); or, after marking, it is edited, determined not to be a dupe, and then this question is irrelevant.
So, I say: Don't proceed the scrutinize the question too much if you notice it's a dupe.
Now, if the dupe is difficult to read and understand; or the original itself was also downvoted (e.g. no effort shown on user's part) or closed (off-topic, opinion-based); or has images instead of code; or other blatant examples of low-quality posts - in these cases - still don't downvote. Make a comment explaining the problem and requiring changes. Only then, if the poster does not address the issues brought up in comments - then you downvote.
Very recently, with MUCH research, on answers that did not overlap my question and dealt mostly with other tangents (eg. jQuery) I received an elegantly simple, practical and knowledgeable answer that did address my situation and solve the problem. The question was marked 'Duplicate' which also devalues the clarity and situational specifics of the answer. Down voting would simply exacerbate trashing a great and to the point answer in the melee.