Oftentimes new questions appear that are "very likely" to be duplicates, but which are not addressed as that (either voted to close for other reason or answered). I would like to "gently remind us" that duplication is an important reason for closure. When navigating past questions, it is not uncommon to find more or less the same question twice (or thrice), which means moderating users might have not pursued a duplicate option (or they might but it did not succeed; not sure we can tell).
Also, we should remember than suggesting a duplicate does not necessarily mean it is actually a duplicate. By flagging as such you are just sort of saying "I think this is a duplicate, please confirm". Other users will check that for you and the asker will explain further why it might not be. And if it is, this could be beneficial for askers who come for quick answers rather than to contribute to the community via building a high-quality, comprehensive Q&A site.
An example of what I mean are this and this new questions. The first one is flagged as "unclear...", whereas the second got two answers from high rep users. To me, viewing such questions raises immediate suspicion that in two and a half year of site activity no one has come up with such a question. Of course it is possible that in fact such exact or very similar question might not exist (a terribly low hanging fruit), but my point is that we should be sceptical that they don't, particularly with broad, basic questions.
I think we agree that flagging as duplicates is time-consuming, and it involves searching for duplicates, and might require some knowledge of the questions' database. But in my experience a one-minute search suffices to find a duplicate with a decent level of accuracy.
Bottom line is, I think we should develop a "duplicate biased mentality", particularly of basic and general questions.
PS: I speak in first person plural because I have probably committed the same fault more than once.