This is not an error of duplication. The full-fledged version of this Mitzvah are many and change for those who are learning the one/three chapter a day.
Positive Mitzvah 212
To Be Fruitful and Multiply
Genesis 1:28 "Be fruitful and multiply"
HaShem created the world for people to live in.
He wants the Jewish nation to grow and expand, ensuring the future of many generations of people to follow the Torah.
This Positive Mitzvah commands us to have children.