Why does the Torah employ the word "la'Shav" twice?
Seforno: The first refers to swearing falsely, the second, to swearing in vain, even if it is true.
Targum Yonasan: Both pertain to swearing in vain. 5
Such as swearing that a pillar of wood is made of wood, or that one of stone is made of stone (Rashi).
That a stone pillar is made of gold (Rashi).
Ramban: To swear that a marble pillar that is located in view of the person swearing is made of gold or of marble.
See Na'ar Yonasan.
What does "Lo Sisa" mean?
Why do we learn from "Ki Lo Yenakeh"?
Seforno: One who swears in vain, he is displaying disrespect towards Hashem's Name - which should only be used to confirm something that cannot otherwise be confirmed. He will not be let off the hook for his display of disrespect. 1
Hadar Zekenim: One who steals, he can return the theft and clean himself. One who swears falsely and desecrates Hashem's name has no way to clean himself. 2
Why are "Lo Sisa", "Zachor Es Yom ha'Shabbos" and "Kabeid Es Avicha ve'Es Imecha" incorporated on the first Lu'ach?
Rashbam: Because honoring Shabbos and honoring one's parents, 1 like not swearing falsely or in vain using Hashem's Name, are all connected to the Kavod of Ha'Kadosh-Baruch-Hu.
Ramban: Just as it is not befitting to share Hashem's Kavod with others, 2 so too, is it befitting to ascribe Kavod to the Name of Hashem, and someone swears by it or who mentions it in vain, desecrates it. 3
Why does the Torah say "Ki Lo Yenakeh" and not just "Ki Yifkod Alav"?
Ramban #1: Refer to 20:7:4:2**.
Ramban #2: To counter those who think that swearing in vain using Hashem's Name is not such a serious offense and is worthy of Divine Pardon.