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 109
Immersing in a Mikveh
Leviticus 15:16 "He shall bathe all his body in water"
Just as the Torah defines the different types of impurity - it also outlines the process for purification.
The Torah describes the purification process, commanding the impure person to immerse himself in a mikveh.
A mikveh is a special pool of water of a specific size.
Its water must come from a natural source, rain or a spring.
In the beginning of the universe, the entire creation emerged from water. After an impure person dips in the mikveh, he rises up as a new creation; purified and prepared to serve HaShem.