Is the 'nonce' in bitcoin really a nonce? Is the 'nonce' in bitcoin really a nonce? Reading definitions of nonce and salt it seems the number is used as a salt but the same value can occur in multiple blocks; there's no requirement or indeed method to enforce that the value is not used again.
I will assume you mean the nonce that bitcoin miners iterate. Depending on e.g. wallet software other salts or nonces may be involved.

That nonce is only used once, because every miner1 will be hashing a different transaction block (hash) – one which sends the reward for solving the proof of work to their address. A miner could reuse a nonce but they would be recomputing something they have already done so it would be a waste of time.

Since the nonce is only 32 bits, miners will eventually reuse values, but that will be with a new timestamp, or even a new previous-block hash (when someone else solved a block).

1 Slightly more complex for mining pools, of course.