Is it legal to copy an NFT?

I'm just wondering, is it actually legal to copy an NFT? I've heard people talking about minting duplicates or even creating their own versions of popular NFTs, but isn't that sort of like stealing? I'm just trying to understand the legalities behind it all. After all, these NFTs are supposed to be unique and one-of-a-kind, so what's the line between copying and actually owning your own legitimate piece? I'd appreciate it if you could clarify this for me.