Disclaimer: I only use SO from time to time, so this is only based on a "light" view of it.
Also, this is not a question about "should we close as a duplicate?". The answer is obviously yes. This question is more about the feasibility of managing all duplicates, and the implications of that.
SO generates a lot of traffic. Questions on the front page stay for about 15-30 minutes max, during which most of them get around 3 answers. It is hard to answer a question without checking new questions very often, since they get answers so fast.
It works well this way; most questions (apart from exotic ones) get correct answers, very fast.
Many questions are duplicates, but only a few seem to be "regular" or obvious ones. In most cases, it will take longer for someone to find a duplicate question, and vote for it, than for someone else to give a correct answer in the meantime.
This is a question only about Stack Overflow, because the other trilogy sites don't have such a large volume of users. On the other sites, typically Super User, duplicates are found, and closed quite fast, most of the time before an answer falls on them. But this is possible only because of the much smaller volume of users and questions.
So the question is: are these duplicates a problem which is out of control when these sites are gaining such volume?
Is it something to really worry about, or does it work fine this way? People can come and get an answer very quickly, without prior searching, just by asking. For most questions, there will always be people to answer. So from the point of view of someone coming to ask, it is not an issue. For obvious reasons, it is also not a problem for people answering, they get reputation this way.
It may be an issue from a storage point of view, since clearly duplication means more redundant data. But more importantly, it can be an issue to find an answer without asking a question yourself. With many duplicates, multiple useful answers become distributed over different questions, and it becomes harder to find most of them.
Extra question following on from these thoughts: should we be worried that the volume of users is finally pushing the site to be much easier to get duplicate answers from, than to search from?