How Is the Uniqueness of an ERC-721 Token Enforced on the Blockchain?

Uniqueness is enforced by assigning a mandatory, distinct tokenId (a unique 256-bit integer) to every single token created within a contract. This ID is immutable and serves as the primary identifier.

Additionally, the combination of the contract address and the tokenId creates a globally unique identifier for that specific asset, ensuring no two tokens are exactly alike.