ذَٰلِكَ الْكِتَابُ لَا رَيْبَ فِيهِ
That is the Book; no doubt is in it.
Under the parsing that treats “dhālika al-kitāb” as a complete assertion, “lā rayba fīh” excludes doubt regarding that identified Book. The close relationship between the propositions makes separation appropriate.
This is not an example of mere word repetition. The later statement adds assurance and strength to the first. That is semantic connection, fundamentally different from the lack of any link in complete disjunction.