Why would you let someone use it twice?
The argument everyone else will be using is, "Why would he let someone use it at all?"
Stop pretending to be so clueless.
An unverifiable system is really not workable. What happens if two people present him with the same code at the same time?
Say one person presents him a claim code. Then a few days later, someone else presents the claim code and has a pile of other proof that he held the asset. How can he know if the same claim code was given to two people or the second person is a scammer who gave the claim code to the first?
He is being compelled to accept liability for a scheme whose reliability he has no way to assess.