Charles Ellicott Commentary 1 Kings 12:33

"And he went up unto the altar which he had made in Beth-el on the fifteenth day in the eighth month, even in the month which he had devised of his own heart: and he ordained a feast for the children of Israel, and went up unto the altar, to burn incense." — 1 Kings 12:33 (ASV)

So he offered upon the altar. —The repetition of this verse is accounted for by its belonging properly in sense to the next chapter, opening the story of the mission of the “man of God from Judah.” The idea of the verse would be best conveyed by rendering the verbs of this verse in the imperfect tense: “So Jeroboam was offering,” etc.