And any place where it comes to impose a condition on the previous negation, it will indicate the condition with the words כי אם (לא אשלחך כי אם ברכתני – I will not let you go, except you bless me⁵³⁴). And with this, it is not the custom to mention the verb again. And when it does repeat the verb, it comes to teach one not to err in thinking that it means that it is optional (reversing the warning that precedes it), but that it is an obligation. Or it is repeated for a special imperative.

Leviticus 21:1-3 says,ויאמר יהוה אל משה אמר אל הכהנים בני אהרן ואמרת אלהם לנפש לא יטמא בעמיו. כי אם לשארו הקרב אליו לאמו ולאביו ולבנו ולבתו ולאחיו. ולאחתו הבתולה הקרובה אליו אשר לא היתה לאיש לה יטמא (And the LORD said unto Moses: Speak unto the priests the sons of Aaron, and say unto them: There shall none defile himself for the dead among his people; except for his kin, that is near unto him, for his mother, and for his father, and for his son, and for his daughter, and for his brother; and for his sister a virgin, that is near unto him, that has had no husband, for her may he defile himself), repeating the word יטמא for “defile”. According to Rule 229, this is to teach that it is an obligation for a priest to defile himself for his close kin⁵³⁵.

footnotes: ⁵³⁴ Genesis 32:27. ⁵³⁵ Malbim, HaTorah v’HaMitzvah Emor 8.