The absurd part would be that there are multiple registries, so more than one person can own the same NFT since they are on different systems. Which one is used

The absurd part would be that there are multiple registries, so more than one person can own the same NFT since they are on different systems. Which one is used to prove ownership? The one backed by entity A or the one backed by entity B? P.S. I find value in this tech but it is still in primitive phase in my perspective.