Haha great brain exercise and because this is a though provoking post i will give a counterargument instead, yes what you describe as double posting a content really sounds an awful lot like double expending, but there is a big difference that i want you to consider, in bitcoin double spending is prevented when the nodes come to one consensus and pick which one is the real transaction and which one is the fake, Steem has the same mechanism to prevent such double expending.

But a post or as we call it a proof of brain, have two cases, the first one would be that, when it was published for the first time it did not reached its full potential, meaning a certain group of people saw it but not all of the people on Steemit, obliviously the second time it gets published it would be another group of people that approved and give it value to the post or proof of brain that it did not had before.

The second case would be, when people that saw the post in the first time it was posted feel that they did not rewarded the author enough for it the first time around, be it because the user was low on voting power, or low SP, and now they want to to give the author something more for his work.

As the extreme cases of abuse of users happen, that gray area of the "permissible" gets smaller and smaller, should we punish everyone that puts the same set of seven words together? o the one that post five times the same content in a week. Like always everything is about lines people draw in the sand .

Great post anyx, i think the more people know and think how they should approach double posting the better, of course, so we can reach a general consensus and draw lines in the sand, this kind of problem can be resolve by talking about them, and this post got me talking and overall thinking about it.