Dueling shouldn't be humorous - particularly when the other person involved in the duel is a dark wizard. A pitched battle, involving several dark wizards ambushing just a few members of the Order should be even less so. And John believes that, up until he sees Sherlock and Sirius fighting at the same time.

As it turns out they never stop competing.

“Expecto Patronum,” he says, in a clear, deep voice, and a thin silver light streams from his wand, expanding to take the form of -

“Huh,” John says, as Sherlock drives the Dementors back.

"Come on," John said, "You can't tell me no one's ever thought to try that."

“Get up, John,” Sherlock says, in a tone that has John jerking upright in bed, immediately, and grappling for his gun in his bedside drawer, even as he says, “Sherlock, what is it?”

Incomprehensibly, Sherlock simply says, “Voldemort.”