I agree. Duality means property of having two levels of structures, such that units of the primary level are composed of elements of the secondary level and each of the two levels has its own principles of organization. Roughly speaking, the elements of the spoken language are sounds which do not convey meaning in themselves. The only function of sounds is to combine with one another to form units that have meaning, such as words. We call sounds here secondary units as opposed to such primary units as words, since the secondary units are meaningless and the primary units have distinct and identifiable meaning. The property of duality only exists in such a system, namely, with both elements and units. Many animals communicate with special calls, which have corresponding meanings. That is, the primary units have meanings but cannot be further divided into elements. Without duality, the communicative power of human language like that of animal language will be highly limited.