A field that you see within a view, such as the Duration field in the Gantt Chart view, is simply a visible copy of the real Duration field that’s located in the database. If you delete the Duration field from the Gantt Chart view, you have removed that copy of the field from the Gantt Chart view only. You have not deleted the "real" Duration field from the database. In fact, not only can you still display the Duration field in other views, you can also display it again in the Gantt Chart view.
