Consider a website where users can post house advertisements for free. Obviously, there should generally be one ad per address. If someone tries to create a duplicate for an address, then they are probably trying to exploit the system by spamming their ad in false hopes of getting more exposure.
Should users be blocked from creating duplicates?
Alternatively, should the user be allowed to create duplicates, but severely penalize the ranking of all their ads?
If either of the above system is implemented and a scenario arises where two people claim the same address, how should a legitimate owner of an address prove that their ad should be considered the "real" ad?