Double-spending already occurs in the world. For example, an author can publish his work in one country in a certain language, then publish it in a foreign country/language with a different language and publisher. This happens already with books, movies, or any other copyrighted material. Even with movies on Blu-ray, they have different distributors across the world, even for the same language content. So I don't think it's a question of ethics to notify every publisher/distributor if you previously printed your work elsewhere. If you're the author, you hold the rights to your work and do whatever you wish with it. The same principle applies on Steemit. Many bloggers earn from other websites, social media, ads, Youtube channels, etc. Steemit is just another media where they can distribute their work and get rewarded. In other words, the distribution isn't exclusive to one place, unless explicitly specified by publishing contracts (a book for example).

And finally, I've seen several users gently get warned by Hive account@cheetah or sometimes flagged by the Hive account@steemcleaners for works they own that was previously posted elsewhere. It's difficult to investigate and determine the validity of claims to 100%, perhaps that's where the ethics may come into play. Is it an impostor stealing someone else's work, or is it truly the person authoring the work.