GEN151 The subject God is repeated twice. The blessing extended to other creatures has but one subject: “God blessed them saying: ‘Be fruitful and multiply’.” Genesis 1:22  The two phrases in the case of man imply a commandment in addition to the blessing. Various commentaries on this verse, including Luzzatto, S.R. Hirsch.  Likewise, in the ninth chapter of Genesis, which recounts the blessing that God bestowed upon man after the Flood, we read: “And God blessed Noah and his sons and said to them: ‘Be fruitful and multiply and fill the earth.’” Genesis 9:1 There is here also a doubling of the phrase as found in the first chapter of Genesis, without the repetition of that subject God.  However, in order to insure against any possibility of error, the phrase was repeated immediately after the prohibition of bloodshed: As for you, be fruitful and multiply; abound on the earth and increase on it. Nahmanides. In light of the above, the Oral Tradition has declared procreation a religious duty, an imperative placed upon man by the Divine Law, a commandment whose purpose is to channelize a wild instinct and subject it to conscious control of man’s intelligence, for the purpose of perpetuating the human species.  ROSNER 63