Blocking duplicates vs penalizing duplicates

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?