Or HaChaim on Leviticus 20:4/אור החיים על ויקרא כ׳:ד׳

1 ואם העלם יעלימו עם הארץ, "And if the people at large choose to ignore, etc." The reason the word "they will ignore" is repeated is to tell us that if the people choose to ignore the first time this abominable crime is committed, they will have paved the way for ignoring all subsequent repetitions of this abomination. Even if they would want to punish people then for practicing this cult, the guilty party would challenge the authorities pointing out that the first guilty party had been allowed to get away with this practice. Alternatively, the first word העלם refers to the failure of preventing the father from carrying out his intention to sacrifice his son to the Molech; the second יעלימו refers to the failure of the people to bring such a father to justice.