The words "Speak to the kohanim…and say to them" seem repetitive. Having instructed Moshe to "speak to the kohanim" about the special restrictions pertaining to them, why was it necessary to reiterate, "And say to them"?
Rashi notes this redundancy and explains that the phrase "and say to them" alludes to another instruction to the kohanim. Namely, that they must ensure that even their young children (who have not yet reached the age of personal responsibility) are in observance of the unique kohen laws as well (see Ramban). In Rashi's words, the double expression comes "to caution the adults concerning the minors."
The Hebrew word that Rashi uses for "to caution" is להזהיר,l'hazhir. This word can alternatively be translated as, "to make shine," as in the Hebrew wordזוהר, zohar, which means, "bright gleam" or "shine." Rashi's words thus hint that the obligation of l'hazhir, "cautioning" others from negative conduct, is fulfilled primarily by focusing on their inherent good, nurturing it until you "cause them to shine" from within. Moreover, the word l'hazhir underscores that your concern to teach and caution others will cause you to shine as well. As the Talmud (Temurah 16a) says of someone who teaches his fellow Torah, "G-d enlightens the eyes of both of them."