Correct, this particular diphthong and the substitution of "an" for "a" serve the same exact function, so having both would be redundant. If we were to look at

Correct, this particular diphthong and the substitution of "an" for "a" serve the same exact function, so having both would be redundant. If we were to look at a word that starts with u but doesn't have the diphthong, such as Ubuntu, then "an" is used.