Stack Exchange recently started a collaboration project with the University of Melbourne, in an attempt to improve the automatic detection of duplicate questions.
More information on the project and a call for help from the community can be found here: Project Reduplication of Deduplication Has ...
^^^ took me about an hour to complete annotation of their examples. Hint to reviewers: firmly say no at blatant non-dupes, these seem to be like "audits" used by their system to determine how much your judgement can be trusted
@ThomasOwens I did a lot of "related" too. Wrt "no" it is really hard to tell because many of these are hard to remember, I recall it usually taking something like split second, when you see "of course no" just at a first glance and click and it goes away. I've been also using "really can't tell" quite generously, both when question topics felt obscure and when I had trouble deciding between "dupe" and "related"