Click "Customize-Interior Modeling 2.0" and select a room to enter the Modeling interface. Left-click to select the shape and click the "Copy" button to duplicate the model.
You can also select the model in the general design interface after unlock it, and then click "Duplicate" to duplicate the selected customized model.
*Note: When selecting objects, if the model is not grouped, only the basic elements of faces or lines will be selected (or hold down the Shift for multi-select). If you want to select the whole 3D solid, you could use the left mouse to double-click the model.