Given that AU is an official support channel for Ubuntu, there really is no good way to prevent an influx of duplicate questions, nor should this really be a problem provided that the question is clear enough to answer.
Thus you are spot on, the best way to handle the large amount of questions AU gets is to *mark all questions that cannot be answered with the information as unclear as early as possible. This not only give the questioner a better understanding of the sites expectations, but makes clean-up eaiser
###Similarly, I would argue that just because a questions isn't a perfect duplicate does not mean you should avoid marking it as one.
Part of asking good questions is discussing what you have already tried, and why other references are not working for you. If someone who asks a question that appears to have accidentally duplicated another question, and fails reference that other question, go a head and mark it a duplicate. However, you should also leave them a comment such as:
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question explaining why that answer did not work for you.
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question explaining why that answer did not work for you.