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 127
The First Tithe
Numbers 18:24 "But the tithes of the Children of Israel which they offer as a gift"
We are commanded to give a tenth of all the land's produce to the Levites.
The Levites were not given an inheritance of land as were the other tribes.
Instead, this share of our crop is considered their inheritance.
Since there are other types of "tithes," (which means: a tenth of the amount), this one is called: "The First Tithe."