You are certainly right, the witnesses could implement something. The problem I have I that there is no way that this could maintain the integrity of a decentralized blockchain. Steem already has weak points with DPOS consensus, this would be a whole new attack vector.

What constitutes a duplicate to some may not to others. I think it is important to keep the blockchain free of as many of these checks as possible and let the actual economic incentives do the work for us. If posts like that become so low value that they are harming the platform, well then some stakeholder is going to have a bot that downvotes them.

Trust in the economics and game theory - if you make it more complicated, there are more points of failure. The economics are extremely complex as they are.