The Duplicate Faces action copies each selected face and either creates a new Mesh or leaves it in the same GameObject as a sub-Mesh, depending on the default options.
From the main menu, select Tools > ProBuilder > Geometry > Duplicate Faces.
By default, when you duplicate a face, ProBuilder copies it into a new GameObject. However, you can choose the Submesh option from the Duplicate To drop-down menu to save it as a sub-Mesh in the same GameObject.