Adding to Tobias' plea for better explanation of duplicate flags, I suggest we give an explanation more often (whenever we can?) for any close-votes.
Sure, we already have "canned" explanations that come with a close-vote, but since they are necessarily phrased in general terms, the OP may have difficulties to understand what specifically makes their question (for example) opinion-based.
In fact, we should always be able to explain how the general close-reason relates to the specific question. Such a reason-giving requirement would not only "be nice" to new users, it would also help safeguard borderline off-topic questions and make us less "trigger-happy".
I sometimes find myself wanting to close questions for being "too broad" just because I can't think of an informative answer, or for being "unclear" just because I don't quite understand what the issue is. But that doesn't mean that someone with better expertise cannot give a useful answer to a somewhat broad or (to me) opaque question. If others here have similar impulses, it might be a good idea to ask for specificity or clarification, wait if edits are made or useful answers are given, and only then, after a while, decide whether voting to close is actually helpful.