﴿كَلَّا سَوْفَ تَعْلَمُونَ ثُمَّ كَلَّا سَوْفَ تَعْلَمُونَ﴾
No indeed! You will soon know; then, no indeed! You will soon know. (Al-Takāthur 102:3–4)
The basic warning is reiterated, but the recurrence adds force to the warning and rebuke. After hearing the first statement, the listener is made to face the consequence again. The added wording thus performs a recognizable function.
The particle thumma also matters. It presents the second admonition as a subsequent stage, allowing a sense of heightened or progressively renewed warning.