The verse already said "v'Yadata ha'Yom." What do we learn from "va'Hashevosa El Levavecha"?
Chafetz Chaim (Zecher l'Miryam 21): "V'Yadata" is to clarify the truth in the mind. "Va'Hashevosa El Levavecha" is to fix the matter in the heart, and fulfill it in practice
Knowledge with the mind is insufficient. If one does not take the lesson to heart, the heart (alias the Yeitzer ha'Ra) will override one's knowledge (EC), as the Gemara says in Sanhedrin, 106b 'Rachmana Liba Ba'i' - See Rashi there.
Lev Eliyahu (Shemos p.155): R. Yonah lists after this Mitzvos Aseh of remembering Hashem's Chesed, fearing Him, loving Him, and clinging to Him. Through contemplating His greatness and remembering His Chesed, one comes to fear, from fear to love, and from love to Devekus.
What are the connotations of "Ein Od"?
Seforno: All the above is only possible if there is one Creator, who is removed from physicality and above it to the furthest conceivable extent. Only one, because the moment there are two, it means that they are basically different. 1
Targum Yonasan: There is no deity beside Hashem.
As we say in the Shema "Hashem Echad".