Tafsir of At-Takathur 102:4

(Then, nay, you shall come to know) is a repetition for emphasis, and "then" (thumma) is used to indicate that the second [statement] is more intense. It is as if the Almighty says to His servant: "I tell you, and then I tell you again, do not do it." It has been said that because it is more intense, it is treated as a distinct entity; therefore, it is connected by a conjunction. Otherwise, an emphatic statement is not conjoined to its own emphatic repetition due to the extreme connection between them. However, you know that the prohibition of such a conjunction is the opinion of the linguists, while the exegetes and grammarians have stated the contrary.

Ali ibn Abi Talib—may God, the Exalted, honor his face—said: The first [mention] refers to the graves, and the second refers to the Resurrection; thus, there is no repetition, the sequence (tarakhi) remains in its literal sense, and there is no issue regarding the conjunction.

Ad-Dahhak said: The first is a rebuke and a warning to the disbelievers, and what follows is for the believers; this, however, is contrary to the apparent meaning.