«وَمَا يَتَّبِعُ الَّذِينَ يَدْعُونَ مِنْ دُونِ اللَّهِ شُرَكَاءَ»
“Those who call on partners besides God do not in fact follow real partners.” (Yūnus 10:66, excerpt; contextual paraphrase)
«إِنْ يَتَّبِعُونَ إِلَّا الظَّنَّ»
“They follow nothing but conjecture.” (Yūnus 10:66, excerpt)
The first portion challenges the reality of the alleged partners; the second explains what the worshippers actually rely upon: conjecture. Repeating the verb “follow” makes the argumentative contrast prominent.
Here “in” has negative force, and “illā al-ẓann” restricts the source of their belief to conjecture. Repetition develops the logic rather than merely saying the same thing again.