For a SECOND time, they designated David’s son Solomon as king

[tonychanyt]

1 Chronicles 29:

21 The next day they offered sacrifices and presented burnt offerings to the LORD: a thousand bulls, a thousand rams, and a thousand lambs, along with their drink offerings, and other sacrifices in abundance for all Israel. 22 That day they ate and drank with great joy in the presence of the LORD.

That was a public affair.

Then, for a second time, they designated David’s son Solomon as king, anointing him before the LORD as ruler, and Zadok as the priest.

Benson explained:

The first time was, when he was made king during Adonijah’s conspiracy (1 Kings 1:34) on which occasion it was done in great haste, and in the presence of only a few of David’s servants; but now in the presence of all the great men of Israel, the princes of the tribes, the captains of thousands and hundreds

The public celebration continued:

23 So Solomon sat on the throne of the LORD as king in place of his father David. He prospered, and all Israel obeyed him. 24 All the officials and mighty men, as well as all of King David’s sons, pledged their allegiance to King Solomon.

The first anointing of Solomon as king was an emergency operation to counteract Adonijah's conspiracy. The second anointing was an extravagant public celebration.