There can be extra duties if you are invigilating people with extra time and additional needs. For example, some students will need the questions read out to them, and certain medical conditions may necessitate a pause. e.g. my younger daughter is Type 1 diabetic. If she were to have a hypo mid exam, she would need to take treatment, and the clock might need to be paused until she is able to continue the exam. She usually takes exams in a separate room due to the potential for her equipment to alarm etc.
My MiL usually chooses to invigilate additional needs if it's an option as she has a background working with children with special needs.
