If you need to duplicate a paper size to showcase a different design then you will need to:
1. Select the paper size group layer and go Layer > Duplicate Group.
2. Select the new duplicated group and expand it.
3. Select the highlighted red layer called ‘Add Design Here’. Control click / right click this layer and select ‘New Smart Object via Copy’.
4. This will create a copy of the layer which you can then add your design to by following the step above. You can then turn off or delete the original smart layer.