The process to duplicate a piece of furniture is the same in 2D and 3D.
To duplicate a piece of furniture, select the item (a selected piece of furniture will have a green circle around it). Click on the Duplicate icon.
The item will appear next to the duplicated item.
All you need to do is move it to the desired location.
Tip: The shortcuts Ctrl+C & Ctrl+V or Apple+C & Apple+V also work.