אלשיך — אמר אל הכהנים בני אהרן ואמרת אלהם / לנפש לא יטמא בעמיו

Q — Why the famous double expression 'אמר... ואמרת'? And why does the verse open in plural ('הכהנים בני אהרן') but shift to singular at the end ('לא יטמא')?

A — The doubling is a הוראת דורות in how to teach Torah. A teacher who has spoken to the public may rationalize: 'I have warned the many; whoever listens listens, and whoever doesn't is responsible for himself.' The Torah corrects this. 'אמור אל הכהנים' — speak to the collective. 'ואמרת אלהם' — and then return and speak to each individual who still needs the warning. The singular ending 'לא יטמא' confirms it: the teacher must come down to the level of the יחיד. The reason this is taught specifically at the kohanim — the most stringent group — is that being told you are subject to restrictions others are not is psychologically the hardest assignment to accept, so it requires the most individualized pedagogy.