That's a very interesting way of looking at it. I'll have to give it some thought. Off-hand: The key distinction between copying and spending is that when something is spendable, only one copy should ever exist, and only one person can utilize it at a time. In contrast, there are many legitimate reasons for copying some forms of content. For example, I've heard a rule of thumb that in marketing, you need to get something in front of people seven times before they will respond to it. One pass through the feed simply isn't enough.

Ideally, I suppose you'd have one copy, and it would pay out for every bit of value that it creates throughout its entire lifetime. With the seven day payout window, though, that's probably not achievable. Is Steem's job to measure the lifetime value of a post, or just its value during a particular time period? I'm not promoting this perspective, but if it's a point in time snapshot, then it could be argued that there's nothing wrong with reposting one's own content. When the flags outweigh the upvotes, the author knows it's time to retire the content.

It's going to get even harder to recognize duplication when some app comes along that uses encryption or some other sort of encoding before posting on the block chain. I assume that's just a matter of time.

I didn't make the connection to double-spending, but I posted a somewhat related article a couple weeks ago.