Definitions of duty:

• noun:   work that you are obliged to perform for moral or legal reasons
• noun:   the social force that binds you to your obligations and the courses of action demanded by that force

  Example: "We must instill a sense of duty in our children"

• noun:   a government tax on imports or exports
• name:  A surname (rare: 1 in 50000 families; popularity rank in the U.S.: #5277)