What does national double patent mean?

There is no such term in official and legal terms. It's just a term created by some companies to promote their products, which makes people feel that their products are very good. In fact, it is possible that he did apply for two patents for this product and obtained authorization, but it is nothing special that one is an invention patent and the other is a design or utility model patent. There may be more than two patents for products like refrigerators that we usually use.