וְאַתֶּם פְּרוּ וּרְבוּ שִׁרְצוּ בָאָרֶץ וּרְבוּ־בָהּ׃
Be fertile, then, and increase; abound on the earth and increase on it.” (Ber 9)
Why the repetition of רבו - increase?
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Rav Hirsch explains that the second רבו is a modification of the שרצו which is a new commandment over and above the basic fruitfulness of the world.
As Rashi says:
And you, be fruitful and multiply: According to its simple meaning: the first [mention of this expression] (verse 1) was a blessing, and this [mention] is a commandment. According to its midrashic interpretation, [it is written here] to compare one who does not engage in propagation to one who sheds blood. — [from Yev. 63b]
Rav Hirsch comments:
שרצו בארץ ורבו בה - a highly significant addition to the mission of development given here to the Noachian world with which this whole new constitution of humanity concludes. Its meaning is given by רשבל in בראשית רבה by the explanation ברית נחלק לאוירות, that Hashem gave a special covenant, a special dispensation, to the different climates and countries. שרץ, as already shown, is the name given to the lowest stage of life, according to the essential sign of its exitence, movement, ישרצו המים. Inasmuch as here the procreation and propagation of the human race have this designation added, the term would represent a swarm of the most manifold different kinds of men,. It would accordingly be a description of a diversity and infinite variety of human races, and moreover בארץ, on the earth, and by the earth, under the influences of the differences of the various lands. And not only שרצו בארץ but also רבו - the whole physical, moral, and intellectual education - equally בה, under the same influences.