ثُمَّ كَلَّا سَوْفَ تَعْلَمُونَ
In the Qurʾān it is not frequent for a sentence to be repeated word for word one directly after the other. The exegetes read this repetition in two ways:
The first reading — emphasis (taʾkīd). The same threat is said a second time, because once is not counted enough. In Arabic, verbatim repetition is one of the strongest means of emphasis. On this reading thumma conveys no order in time; it conveys an order of degree (the usage the lexicographers call tarākhī fi'l-rutba): the second warning is heavier than the first.
The second reading — two separate times. The first "you will know" points to the moment of death or to the grave, the second to the resurrection and the raising. On this reading thumma conveys a real order in time, and there is a long interval between the two knowings. This view too is widespread in the classical commentaries, and it agrees with the second verse of the sūra: a visit to the graves had been spoken of, so something happens in the grave.
Both readings can be defended. The second appears to sit a little better with the flow of the sūra, because the text is already setting up a timeline: the race → the grave → ? → the seeing of al-Jaḥīm → the questioning. But this is a preference; there is no proof that weakens the first reading.
The repetition has a further effect, independent of meaning, and it has to do with the rhythm of the text.
The first two verses are fast and dense: a diagnosis, a limit. The third verse cuts. The fourth verse, by repeating the cut, brings things to a stop. The text is not advancing; it is standing in the same place. The reader waits for the advance and the advance does not come.
This is the text keeping the one addressed waiting. And waiting is what is needed for the threat to work — because what is threatened has already been thrown forward into the future by sawfa. The sūra does with its form what it says in its words: while postponement is being described, it is postponing.
One further observation: kallā occurs three times in the sūra (verses 3, 4 and 5). Three verses of an eight-verse text begin with this particle. The middle of the sūra consists of three rejections laid one on top of another.