Or HaChaim on Deuteronomy 6:4/אור החיים על דברים ו׳:ד׳

1 שמע ישראל ה׳ אלוקינו ה׳ אחד. "Hear, O Israel: The Lord our G'd, The Lord is One." Why did the Torah repeat itself instead of writing ה׳ אלקינו אחד, "the Lord our G'd is One?" The reason is that the Torah [or rather Israel in its declaration of faith, Ed.] referred to two aspects of G'd. 1) He is our G'd; 2) He is One, i.e. unique. This means that we accept G'd as our exclusive G'd even assuming there were more than one G'd in the world. We accept Him as the Tetragram with all that this implies regarding His various attributes. Furthermore, we express our conviction that He is indeed the only G'd, there is no other independent power in the universe. This by itself would be sufficient reason to serve Him even if He did not represent all the virtues we ascribe to Him.

2In other words, the thrust of this declaration of our accepting the rule of Heaven is that either of the two aspects of G'd, i.e. אלוקינו or אחד is sufficient by itself to establish our relationship with Him. If the Torah had not phrased our declaration of accepting His rule by repeating the word י־ה־ו־ה but had written ה׳ אלוקינו אחד, the implication would have been that the reason we accept G'd is that there is no alternative to Him, He being the only One. In such an event one cannot claim credit for serving the one and only master. By expressing this declaration in the manner recorded in the Torah, Israel makes plain that our acceptance of Him is not one that is due to a lack of choice. We have additional reasons for accepting Him, over and above the fact that He is indeed the only Master.