duty是职责之内的,不一主观条件为转移的,而obligation则强调自己自愿承担的责任.
对比下面两句话:
It's my duty to clean the classroom.
It's my obligation to clean the classroom
前者强调不得不承担扫教室的责任,后者强调我自愿承担扫教室的责任.