Legally, there’s nothing stopping you. The only thing that would make “A” the canonical NFT for “X” is social convention. I imagine that where the original creator of the artwork is involved in the minting of X that convention can be pretty strong, but I also expect lots of boundary-testing. This is the art world after all, and a particularly weird slice of it at that.
Actually, legally, there is. Copyright still exists, and still applies to NFTs.
Sure, in the same way that copyright still applies to NES roms. That doesn't really stop anyone from copying them and calling Nintendo sill for asking $40 for one.
And does copyright prevent someone from sharing a link to someone else’s work? If so, someone should tell Google.