I dont see the problem of double spending in the form you describe it.
Lets say someone posts his own post a second time. Yes, he received upvotes for it at the first time already, but not from the same people - assuming that we dont have a lot of steemians here suffering from memory loss.
So those who upvote the second time posting, have not double voted for the same content. They only didn't see the first time post and missed to upvote it then.
That situation even is in effect if someone steals the content from someone elses post. That being said, it s cheap and unethical to do that of course (and a possible copyright violation). But even then, the people who dont know that its stolen content and like and upvote it, have not double spend on the same content.