The uniqueness of NFTs is guaranteed by using blockchain technology. Each NFT is created with a unique identifier and is stored on a decentralized network of computers, such as Ethereum. This means that the ownership and authenticity of the NFT can be verified by anyone on the network, making it impossible to duplicate or counterfeit. Additionally, smart contracts can be used to enforce rules around the use and transfer of the NFT, ensuring that it remains a unique and valuable asset.