Duty is a task that you are expected to accomplish; it is a requirement of the situation and something which every one must do. If you are a baker, you must come to work on time, complete your tasks and act responsibly to customers and coworkers. These are some of the general duties of a baker.
If these duties were to be totaled; this would constitute your service as a baker. Service is the compilation or total of a variety of duties and usually described in relation to other people. Therefore you bake bread for others to eat.
Service on another level relates to duties which are communicated and carried out on a spiritual plane. When one is in service to God/Light, the personal will is stilled and the servant acts in accord with the Master’s Desire. Here the servant and Master are One, and the duties which are performed, constitute- the Higher Service.
Check-out my two books: Sufism for Western Seekers: Path of the Spiritual Traveler in Everyday Life and The Ferryman’s Dream. Both books are available through Amazon.com or local bookstore.