Is there a repetition here? No, there is not any repetition in the Holy Qur’an and it is not a repetition for emphasize any some scholars said because Allah (swt) joins the two by (Then):
ثُمَّ (Then): This is a conjunction which brings the meaning of a prolonged period of time.
Therefore, they will come to know two times.
There are “Ilmul Yaqeen” which means ‘the certain knowledge’, “Ainul Yaqeen” which means ‘the certainty of sight’ and the “Haqqul Yaqeen” which means ‘the certain truth’.
Therefore, “كَلَّا سَيَعْلَمُونَ” (No! They are going to know ) means that they will know with the certainty of sight when they themselves will see the Paradise and Hellfire and:
ثُمَّ كَلَّا سَيَعْلَمُون (Then, no! They are going to know ) means that they will know with the certain truth when a group of them enter into the Hellfire and another group into the Paradise.
So, there is no repetition here. Rather, there are two stages, the stage of ‘the certainty of sight’ and ‘the certain truth’.
Directly after this, Allah (swt) moves from the topic of the Hereafter to the signs of the universe. It is as if Allah (swt) is saying that the signs in the universe are in front of you, mankind, to assert to you what is hidden from you, so I give you the evidences that are available to you, the signs of the universe that affirm the existence of The Creator (swt). Thus, if you have reached the belief that the Creator is Allah (swt), you will attain the belief in the Day of Judgement without having any doubts.