Let's take two words:
As I can see it that they are identical. They aren't two different words that have the exact same meaning, in fact "uttermost" is just another word for "utmost" and the only difference I see is the different prefixes "ut" and "utter". So conclusively these two words are in fact clones.
Prepositions and Particles in English: A Discourse-functional Account By Elizabeth M. O'Dowd says that the prefix "ut" is - An Old English prefix that means "out", confers a resultative state on its completement: we understand that "to utter" means to put words out, so our attention is directed to the output, as well as to the act, of speaking.
I can't find the explanation for the prefix "utter".