I'm a little hesitant to offer another answer, because my answer is not as germane as some of the answers already here. But it is yet another perspective. Much of what is perceived as toxicity is really due to the following:
There is a disconnect between the newcomer and the mentor who offers coaching on the question.
Mentors can be people who offer advice on rephrasing a question so as to improve its quality, or who flag a question as a duplicate, or who downvote a question and comment on the reason. Just about anything other than offer an answer.
Most mentors intend to do good. sometimes it's more about what's good for the community than about what's good for the newcomer, but often it's both. But it's very easy for the most helpful of mentoring to be perceived as elitism, rudeness, or condescension.
Newcomers typically do not perceive of themselves as joining a community. They think they are just asking a question, and they think they know everything they need to know about Q&A itself. They don't, but they don't know what they don't know.
Newcomers often think of themselves as valued professionals who should be welcomed as colleagues, and are completely unprepared for what I'm referring to as mentoring. They think of it as an obstacle to getting a good answer, not as a way to move towards getting a good answer. And they think of the answer as being good or bad solely in terms of whether it helps them individually, not as something that might be valuable to future visitors.
The problem for the seasoned regular is how to bridge that gap without being too welcoming of questions that will lower the standard set over more than ten years.
Let me add just one little tidbit that I've found helpful.
There are several ways to indicate that a question has been asked before. One is to mark it as a duplicate, indicate which question it duplicates, and start the closing process. Another way is one I like a lot more. It's a link with the following label:
This question may have an answer here
Notice the difference. The first response suggests that asking the question was a faux pas. If the asker had done a better job, the question would never have been asked. Therefore the asker should feel slightly guilty about having violated our norms.
The second form is silent about the question itself. Instead, it suggests that, by following the link, the asker may get what he or she wants. A useful answer. The fact that the useful answer have have already been there before the question was asked is implicit, but not explicit.
The next suggestion I have involves a major effort, one that may be beyond our expertise.
Find a way to make it easier to search for similar questions.
This sound like a problem already solved, but it isn't. I'm going to make an analogy. There is a database of millions of fingerprints, and someone might inquire about finding prints that are somewhat like a sample print that relates to a current case. The problem is that "somewhat like" is actually multidimensional. One of the major advances that the FBI made was to come up with a way of forming a catalogue of fingerprints what would put similar prints close to each other in the catalogue.
If you took the millions of questions we have on file, and had a way of measuring the "distance" between each of those questions and a question that is proposed to be asked, we could point the new user at a list of similar questions.
The problem is compounded by the fact that people who don't know the answer to a question also don't know how to categorize the question. Tags are a start, but I think we need to go way beyond that.
This is way beyond my expertise. But maybe it isn't way beyond yours.
