Ownership of a name is proven by holding a token of its category: an update
is accepted when some output carries it. Nothing required that the
certificate be the only such token. Whoever registered a name could mint a
second one in the same transaction, a minting token for instance, sell the
certificate on the market, and later publish an update the resolvers would
accept. The buyer would hold the certificate and lose the name.
A registration now counts only if its category is created by that very
transaction, exactly one output carries it, and that output is an NFT with
no capability and no fungible amount. Nothing meeting this can ever have a
sibling. The same holds for TLD certificates, whose category proves the
right to set a TLD's policy and to co-sign registrations under it.
One definition, in resolver-web.js; bns.js and the TLD snapshot builder call
it. Every valid registration on chipnet already satisfied the rule: the live
registry is the same 71 names and 21 TLDs before and after.
Found while writing the delivery escrow, which had to forbid exactly this.
