Tadhyīl (التذييل) reinforces an earlier statement by adding a generalizing or confirmatory clause. Repetition (التكرير) more directly restates material to heighten its force. Both can emphasize, but the mechanism differs.
«إِنَّ الْبَاطِلَ كَانَ زَهُوقًا» gives a general claim supporting an event; «كَلَّا سَوْفَ تَعْلَمُونَ ثُمَّ كَلَّا سَوْفَ تَعْلَمُونَ» repeats a warning to intensify it.
In analysis, naming both simply “expansion” misses the essential distinction between general confirmation and emphatic reiteration.