The words “say to them” seem to be unnecessary, as it already says that Aharon was being told what to say when he blesses Klal Yisroel.
My ancestor, Rav Mordechai of Nadvorna zy”a (Sefer Maamar Mordechai) explains that this is meant to teach us that a genuine tzadik who is on a very lofty spiritual level can accomplish great things through the power of his words. He doesn’t need to engage in excessive bakashos or the like. Rather, his words alone can bring about salvations, as “a tzadik decrees and Hashem fulfills.”
Sefer Noam Elimelech (Parshas Shoftim) expounds on this concept and says that there are tzadikim who can accomplish everything through their holy words alone. This is the meaning of the pasuk (Mishlei 12:18): “The words of chochomim heal.” Through words alone, a chochom can bring about a refuah.
Accordingly, this pasuk is saying that Aharon should “speak to them.” In other words, through his speech alone, he can bless them.
דַ ּבֵ ּר אֶ ל אַ הֲרֹ ן וְ אֶ ל בָ ּנָיו לֵאמֹ ר כֹ ּה תְ בָ רֲ כוּ אֶ ת בְ ּנֵי יִ שְ ׂרָ אֵ ל אָ מוֹר לָהֶ ם (ו, כג)
Speak to Aharon and to his sons saying: Thus shall you bless the children of Yisroel. Say to them... (6:23)
יְ בָ רֶ כְ ךָ ה' וְ יִ שְ ׁמְ רֶ ךָ (ו, כד)
May Hashem bless you and watch over you. (6:24)
