What is the ruling when two people give conflicting reports about which vessel a dog licked?

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If one person states a dog licked a specific vessel (vessel A) and the other states it licked a different vessel (vessel B) but not the first, both vessels must be avoided. The statement affirming an event is accepted over the statement of negation, as it is possible for each informer to know what the other does not. This acceptance holds unless they specify the same exact time and the same single dog, such that the time is too short for the dog to have drunk from both, in which case their conflicting statements nullify each other, and using either vessel becomes permissible.