كَلَّا سَيَعْلَمُونَ
Compare 78:4 and 78:5
highlighted words differ between verses
ثُمَّ كَلَّا سَيَعْلَمُونَ
Consecutive verses both reading 'kalla sa ya'lamun' (No — they shall come to know) — an exact verbatim repetition used for rhetorical emphasis within the same surah.
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