Copy and paste
Copy and paste icons can be found in the top left corner next to the name of your project.
(Shortcut CTRL + C)
Select what you want to copy and click on the copy icon or press CTRL+C.
(Shortcut CTRL + V)
Click on the paste icon or press CTRL+V. The selection you copied will be duplicated.
Tip1: You can use the function Extrude for duplicating the whole object. Select the whole object, press SHIFT and drag the transform gizmo.
Tip2: You can duplicate the whole Layer. Go to the Layers in the bottom left corner to duplicate the Layer