You seem to be suggesting that a problem with NFTs is that the artist can mint multiple copies of their artwork, possibly using secret additional private keys.

You seem to be suggesting that a problem with NFTs is that the artist can mint multiple copies of their artwork, possibly using secret additional private keys. The answer is that this is not a problem, because the whole point is knowing who the creator of the NFT is, and beeple trying to create additional copies using a secret wallet but not telling anyone will be treated by the market the same as me minting a copy of a beeple piece.