Ok, yeah, that's what I thought. So someone could sell a multimillion-dollar NFT, then someone else could duplicate it manually and "re-sell" it, and if they didn't do proper due diligence...
This is was enables art to happen: https://smart-contracts.host/
But on the blockchain itself, the NFT is just a smart contract (bit of code, bit of data) that knows it's current owner, it's name and a URI pointing at the image. There zero mechanism preventing duplicates.
Hell, it's such an unregulated market that the NFT might be based on a custom smart contract with a backdoor that allows the creator to steal it back at any time.