The Motive of Duty

This week in class we discussed the multiple meanings of duty. Kant believes that duty is something that you ought to do and he goes on to say that there is a motive of duty. The motive of duty says that a human action is morally good, not because it is done from inclination or self interest, but because it is done for the sake of duty. I feel like this description is true for most actions; however I disagree with it because I feel like there are times when a person performs an action that is morally good and also has a personal benefit from the action. Does the fact that the person received personal benefit from the action automatically make the act immoral? I do not believe that it does. For example, if a fireman went to rescue a person from a burning building he is doing a morally good action for the sake of duty, but he is also receiving personal benefit from the action because he is receiving a paycheck for being on duty at the time. Just because the fireman got paid does not mean that the action was immoral. Another example where this would apply is when a teacher helps a student with a project or helps the student learn a difficult lesson. The teacher that is helping the student it is their duty to help and it is the morally right thing to do and the teacher is also receiving a paycheck. So the teacher is still receiving personal benefit from the action but I would still consider it to be a moral action because it is done to help. Therefore I believe that in some situations that even if a person receives personal gain from a situation does not mean that the act is immoral.