This function is a little bit of a hidden feature.
Drag & Drop duplication
You can drag a box from the grid to the top half of the container and boxes toolbar. If you drop the box on the droppable “duplicate” area this box will be duplicated below the original box.
Alt key duplication
If you hover over a grid box you can see the context menu with the drag icon, edit- and delete-button. If you hold alt on your keyboard the edit-button will switch to a duplicate button. The duplicated box will be placed below the original one.