Could you please explain one thing: 
If I buy an art NFT, do I own a copyright of it? 
May I use it in commercial purposes? 
If there are multiple NFTs of the same pic, are the rights shared?

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Short answer: could be anything/nothing

The NFT creator (copyright holder) defines what rights the token represents, usually in a license agreement tied to the contract or in the metadata.  
The rights could be very broad or restrictive, usually following the general categories of rights used in IP law.

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In many countries the creator also has certain inalienable rights to the asset that can almost never be sold/superceded such as moral rights that may limit usage of the asset.

Sorry if this was much to deep an answer!

The creator only confers rights to the underlying asset (art, tickets, metaverse property, game item, etc) via the token such as display rights (commercial or personal), usage rights, financial uses,  reproduction rights, derivative works, transfer terms (royalties), etc.