Or HaChaim on Numbers 6:23/אור החיים על במדבר ו׳:כ״ג

1 דבר אל אהרון ואל בניו, "speak to Aaron and his sons, etc." Why did the Torah repeat the word לאמור in this verse? Perhaps the Torah was afraid that the blessing described here was something that the priests could pronounce but were not obligated to pronounce. The Torah wanted to make sure that although the word כה at the beginning might be construed as something to be performed voluntarily, i.e. "if you bless you shall keep to this formula," the priests should know it is a positive commandment to bless the Israelites. The repeated use of the word לאמור may also mean that the commandment did not only apply to Aaron and his sons but to the priests in all subsequent generations. There is also an implied compliment to the priests here, i.e. the priests are privileged by G'd to transmit His blessing to the people by pronouncing G'd's wording.