As I mentioned, the issuer needs to represent if the NFT represents exclusive ownership of the underlying asset. If the issuer says the NFT represents exclusive ownership of an asset, then tries to sell a similar NFT on another blockchain, they are selling something they don't own, and would be committing fraud.
i dont think most people that buy nfts have any type of ownership over the artwork itself thus the above is a mute point. just because there is only one up for sale on some marketplace on some blockchain doesn't mean they can't go and sell the same thing on other blockchains. buyer might not like it but that's how it goes. now i think where it might seem unethical to do is to go and sell it on some other marketplace using the same blockchain. but buyers have no control in any event.
What exactly do you think people believe they are buying when they buy an NFT? Why do you think people have been willing to spend, in some cases, millions of dollars for NFTS? Because they have too much money in their bank account?