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 222
Divorce Procedure
Deuteronomy 24:1 "Then let him write her a bill of divorce and place it in her hand"
The Torah teaches us the proper manner through which a Jewish marriage becomes a union of holiness. If for some reason a marriage must be ended, there are specific laws that must be followed.
This Positive Mitzvah details the manner in which a couple may divorce.