One thing I don't see people mentioning is that there's set-up for devil fruit cloning in One Piece. Oda has used smile animals that we already have Devil Fruits for. Granted smiles are incomplete BUT what happens if a 'perfect' smile is created? We'd have 2 identical Zoan fruits
The main reason smiles are defective is because Caesar made them. Vegapunk however, has already created a perfect artificial devil fruit. Yes, Momonosuke SH's fruit i would argue is actually a replica of Kaido's fruit. Its the reason why we haven't gotten their DF names yet
It's also why Kaido still hasn't seen Momo transform yet. It's interesting because then it implies that duplicate DF not existing is not a STORY RULE, but more so a circumstantial reason. Maybe DFs were only created once, then their creator died and no one could recreate them.