Furthermore, there could be an added regulation that no two domains could serve essentially the same data for more than a certain transition period, such as three months. Not only are such duplicates bad for spiders, but there is no essential reason why they need to exist. With redirects and load balancers, a site could easily be switched over from one domain to another in a relatively short time.
One of the fundamental rules of this world is that if something can be abused, it will be abused. And unlike infractions that are obvious and hurt a person's social reputation, domain registration is so hidden that a person can squat left and right without social repercussions. Hence, the only solution is regulation -- either technical, or legal.