I'm pretty sure no one of us wants duplicates within the Raspberry Pi Stack Exchange because we all hate redundant work.
Now to the questions that are answered anywhere else on the net, but didn't appear on the stack yet: I think we should keep them.
- In case we want the stack to be a comprehensive and reliable source for expert knowledge about the RPi, sustainability of this data has to be independent of other sources. We all know those frustrating dead links.
- In case we want the stack to be a fast, high responsive Q & A platform about the RPi: The questioner just turned to us with a problem he can't solve with the information available on the stack, so let's solve it.
But I agree, that we need to encourage basic research (and you will never ever get rid of this task). I think best is to lead by example and to point out missing research in an objective and friendly manner.