'Duty' in the Bible

They spent the night around the house of God, for the [night] watch was theirs, and they were in charge of opening the house morning after morning.

Now these are the singers, heads of the fathers’ households of the Levites, living in the temple chambers, free from other service because they were on duty day and night.

For their duty is to assist [the priests] the sons of Aaron in the service of the house of the Lord, in [caring for] the courtyards, the chambers, the purifying of all holy things, and any work of the service of the house of God,

These were their offices and positions for their service in the house of the Lord according to the ordinance given to them by their father (ancestor) Aaron, just as the Lord God of Israel had commanded him.