Or HaChaim on Deuteronomy 27:15/אור החיים על דברים כ״ז:ט״ו

1 וענו כל העם ואמרו אמן, and the entire people are to respond by saying "Amen." In this instance the Torah was not satisfied to write ואמר כל העם as it did with respect to all the other ten curses listed here. The Torah wanted to make certain that the Israelites were required to respond (not merely say). Had the Torah not written the word וענו at this point we might have thought that it was the Levites who had to say the words ואמר כל העם, "and the entire people are to say."