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.

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.