Duplicating is, together with deleting, the two actions you can fulfil on an element from the timeline. The duplicating action can be executed on any element already fixed on the timeline. As opposed to simply dragging again the mentioned element to the project, the duplicating action not only creates a file clone, it also copies all the parameters of its specific configuration, that is to say, it creates an exact duplicate of the element and locates it on the same position and on the layer immediately above the original element.
Depending on what you would like to obtain, you have the option to duplicate an element or simply import it to the project again. If the parameters you need to change are many, it might be more practical to drag the element again from the Library, while if the changes are few –just one or two parameters, as the position or entry time might be–you can save time by duplicating the element you have already configured.