I feel like we are talking past each other.
Are you saying that 2 entities cannot assert the same thing? Because that does not mirror reality. My understanding of the attestation spec above is that an attestation only can apply to 1 assertion, but if there are 2 assertions that are the same, how would that be handled, considering that a user attesting to the fact that the sky is blue isn't relevant whether God or the devil made that assertion.