StackExchange only flags duplicates, that's true, but the reason is so that search is more efficient, not less. The duplicate serves as a signpost pointing to the canonical question.
Also, StackExchange does indeed allow edits to answers by people other than the original poster. Those with less than a certain amount of reputation can only propose an edit and someone else has to approve it, and those who have a higher level of reputation can edit any answer and have the edit immediately go into effect.
You may also be interested in the StackOverflow Documentation project (now defunct). I think it attempted to do something closer to what you're suggesting.