Or HaChaim on Numbers 8:20/אור החיים על במדבר ח׳:כ׳

1 ויעש משה…וכל עדת בני ישראל..כן עשו להם בני ישראל. And Moses and Aaron and the whole congregation of Israel did….so the children of Israel did to them (the Levites). Why did the Torah have to repeat the line: "so they did?" Why did the Torah speak about the "whole community of the children of Israel" at the beginning of the verse, whereas at its conclusion it only speaks of "the children of Israel?" It appears that the line כן עשו בני ישראל is repeated as it is a reference to the firstborn who are being complimented for agreeing to their new status seeing they were the ones who lost many privileges due to this change. This is also the reason that at the end of the verse the Torah did not speak of "the whole community of Israel," since only the firstborn of the Israelites were meant.