Would most game theorists say that they would defect against copies of themselves in a one-shot PD?
It depends on what “against copies of themselves” means. If it means “I know the other person behaves like a game theorist, and the payoff matrix is denominated in utility,” then yes. If it means “I know the other person behaves like a game theorist, but the payoff matrix is not denominated in utility because of my altruism towards a copy of myself,” then no. If it means “I expect my choices to be mirrored, and the payoff matrix is denominated in utility,” then no.
It depends on what “against copies of themselves” means. If it means “I know the other person behaves like a game theorist, and the payoff matrix is denominated in utility,” then yes. If it means “I know the other person behaves like a game theorist, but the payoff matrix is not denominated in utility because of my altruism towards a copy of myself,” then no. If it means “I expect my choices to be mirrored, and the payoff matrix is denominated in utility,” then no.