Verse 7 already taught not to rebuke a Letz. Why is this repeated?
Orchos Tzadikim (Sha'ar ha'Chanifus DH ha'Shevi'i): If one sees stubborn people and does not rebuke them, for perhaps they will not heed him, he sins. Perhaps they would have heeded him! Total Tzadikim were punished for this at the time of the first Churban (Shabbos 55a). However, if it is known and tested that the sinner hates Musar and will not listen, our verse teaches that one does not rebuke him.
Malbim: A Letz disgraces only one who gives to him Musar, but not one who rebukes him via intellectual proofs. He cannot scoff at what Sechel clarifies. Even so, he does not love intellectual rebuke - "Lo Ye'ehav Letz Hoche'ach Lo El Chachamim Lo Yelech" (15:12). He avoids Chachamim, lest they rebuke him via intellectual proofs. Therefore, do not rebuke him even via proofs, lest he hate you, so he does not love rebuke.
Why will a Chacham love you for rebuking him?
Malbim: He loves Tochachah. This is why the Na'aros sent to invite the Pesi and Chasar Lev to the meal of Chachamim, they call also Chachamim to be invitees, even though they already received the laws of Chachmah.
