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This question is related to the fact that Stack Overflow has a great community that gives insightful answers. Quick.
For many programming topics, I believe the set of all answerable/relevant questions is finite. Because of this the amount of duplicate questions will only increase with time.
It could happen that some day, it will be very difficult to ask a question that has not been answered before.
I don't know what is the influx of users into Stack Overflow, but I assume it is more or less constant, or even exponential. Nevertheless people are born every day, many of them will become programmers, and will end up here searching for knowledge.
One could argue that programming languages get created all the time, and that the problems to solve by programming are endless. Although those two premises are correct, for existing languages acquiring reputation points is still going to be harder.
What I'm trying to say is that reputation points are a finite resource and there's not enough RP for everyone.