Very cool discussion.

I do find a slight possible flaw in your thinking when comparing to the bitcoin blockchain. The bitcoin blockchain has a set amount of rewards created per block. Replaying a block creation would change the inflation rate of the currency itself which would be a very serious thing. In this DPOS system, the 3 second block production is trustworthy and unchanged, regardless of how many double spends of content happen. To me, that's an important distinction. To me, there are two levels. The first level creates the token and that's the really important one that should not be possible to "double spend" or it changes the fundamental economics of the token. On a second layer, we have the proof of brain distribution of the rewards pool. Ideally, yes, we want to avoid double spends here as well, but I don't think it's at the same level of concern as a block production which creates new tokens out of nothing.

As I was reading through this, I was reminded of a post I voted for recently which was a copy of a post which was vendetta flagged to $0 by a whale. It was a valuable post (IMO) and deserved a portion of the rewards pool and since the original version was flagged to $0 for irrational reasons (IMO), I was happy to vote it up. To me, it's an example of how all value is subjective. In that case, I was happy to support it because the author disclosed the duplication (and the reasons for it) and the post was personally valuable to me.

That reminds me of a post I did last year: Why You Should Care About Plagiarism and Fair Use. From that perspective, it's about determining who in the community is a good, trustworthy actor, and who is not. If someone republishes super valuable content, I'm okay with rewarding it if it's new for me and I know this person isn't purposely trying to scam anyone such as reposting something which was supposed to be exclusive content elsewhere.

From a certain perspective, every time a movie plays in the theater, they are double spending the original work. It's not about some labor theory of value, more about who is willing to subjectively assign a value to the exchange in that moment and pay for it.