2 Samuel 20:23 - Albert Barnes' Notes on the Bible

Now Joab ... - This is by no means an unmeaning repetition. Joab had been dismissed to make room for Amasa, and was now, as the result of his successful expedition against Sheba, and the death of Amasa, reinstated in his command. Moreover, this was a fresh beginning of David’s reign, and therefore a statement of his chief officers is as proper as in 2 Samuel 8:16, when he had just established himself on the throne of Israel. Compare 1 Kings 4:2-6.