A basic question about NFTs uniqueness

I have a basic question about the uniqueness of NFTs. At first I thought that all NFT protocols / standards would only allow some data (the content = a picture, sound etc) with the same hash to be owned by one address. This meaning that each NFT minting would only be allowed if the hash of the content have never occured before within the same NFT-protocol. But after doing some research, it seems like content with the same hash can be minted more than once. This I think makes the NFT concept kind of useless. Am I correct, and what are your oponions on this?