serve or act as a substitute for something else
代替…之用，充当
the rusting shack which did duty as the bridge.
当桥用的生锈棚屋。
engaged (or not engaged) in one's regular work
在（或不在）值班（的），在上（或下了）班（的）
the doorman had &B{gone off duty} and the lobby was unattended.
看门人下班了，大厅无人照看。
Obliged
责无旁贷的
You are duty bound to help your little sister and brother.
你有责任帮助你的弟弟妹妹