What the Bible says about Doubling -up as Emphasis
(From Forerunner Commentary)
In this context, light and truth are basically synonymous. The Hebrews had a way of "doubling-up" or "doubling-over" a word, a term, a phrase, so that one would emphasize the other, despite both meaning essentially the same thing. This method hits a concept from two slightly different angles so that it becomes more emphatic. Light and truth are the similar ideas, the word "truth" channeling the author's illustration of light and what it signifies. Light illustrates, emphasizes, and expands the abstract idea of truth.
John W. Ritenbaugh
Truth (Part 3)