If so, then it would actually be legally and morally ok for the artist to do a «double sell» as I describe in my original comment. It would be like i.e. selling a signed copy of a book to one person (the NFT) and the underlying copyright (the sales contract) to another person.
Morally is complicated if they are not transparent about that from the start - the social expectations around this are not fully developed yet, but preventing the holder of the NFT from displaying the work (by selling an exclusive license to somebody else) would probably be seen as wrong.
If there's no legally binding contract attached that you won't do it, then it should be just fine to sell as many instances as you want, no? In the absence of a contract you're just buying a unique tag, not the work itself.
