What interests Kant in all these cases is only this: Did the agents in these cases have to constrain themselves through respect for moral principles in order to perform the dutiful action? If they did, and the agent did the dutiful action, then that action was done from duty. If they did not, then the agent is not acting from duty in the sense intended in this discussion (whatever the real motive for the action may have been – in case that issue were to come up).

Kantian Ethics (2008) - Ch. 2. Moral Worth