Hi Jack Conrad,
Wires don't have to have two layers of insulation. When using ENT (commonly called smurf tube) or gray plastic conduit it just works out that way. But when using EMT, rigid metal conduit, metal flex conduit, or BX there is only one layer of insulation (on the wires). The purpose of the conduit is for physical protection and support of the wires.
Good luck, Sam MC8