Comment
The definitions used here are quite old. However, they have not changed much since the 1970s. In some respects these concepts (duality and productivity) are not especially helpful, because they subsume to simply saying that language is organized at two levels and this allows us to construct novel utterances. As I have indicated, this is fairly self-evident and it does not add a great deal to our understanding of the nature of language. The listing approach is not used much nowadays but it remains a favorite type of question on linguistics courses: rather like the perpetual focus on the Sapir-Whorf Hypothesis. This is the idea that thought is the same thing as language. Whilst it gained much attention when first proposed it has now been debunked, as recent advances in cognitive psychology have improved our ability to study thinking and to separate this process from language. Despite the fact that there is no credible scientific evidence that language and thought are the same thing, this topic keeps on being set as an essay question in college/university courses.