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 154
Resting on Shabbat
Exodus 23:12 "And on the seventh day you shall rest"
Look at a globe or a map and spot an island.
Do you see that bold brown patch of land amidst the surrounding blue water area?
It is clearly visible, existing peacefully and calmly, while the waves fling themselves against its sandy beaches.
The seventh day of the week - Shabbat - can be compared to an island.
Not a geographical one, but rather an "island in time."
The days before and the following week are busy and full of weekday activity. They are like those tossing waves in the sea. Shabbat is like that peaceful island.
We are commanded to stop all our work and rest on Shabbat.
On Shabbat, we do not concern nor involve ourselves in our weekday work and occupations.