serve or act as a substitute for something else
代替…之用，充當(dāng)
the rusting shack which did duty as the bridge.
當(dāng)橋用的生銹棚屋。
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.
看門人下班了，大廳無人照看。
Obliged
責(zé)無旁貸的
You are duty bound to help your little sister and brother.
你有責(zé)任幫助你的弟弟妹妹