There can be a lot of repetitions in elements of tasks or milestones e.g. dates, assignee etc., when building or updating a plan. Duplicate feature is meant to save time doing that.

When clicking on it for the first time, or opening it with keyboard shortcut (ctrl+D/command+D), you will see this modal.

Here you can choose which elements you would like copied in the duplications. After this you can simply use keyboard shortcut (ctrl+D/command+D) to generate series of tasks with the selected element(s) being copied in them. They will be created right below the original task.

Your choices in the modal are remembered in all of duplicate actions you take in Genibelt until you open it again and select different elements.

You can also do it for several tasks from the bulk panel.