I came across this in the first paragraph of an old post and cringed. First, «complete and utter» seems redundant. Perhaps it isn’t always; the two words have somewhat different definitions if very similar usage. It’s possible that one would want to specify both that something is not only totally whatever attribute is being ascribed, but that attribute in the most extreme manner.
I’m having trouble coming up with an example of that hypothetical, which leads me all the more to my original conclusion: this phrase is almost certainly redundant.
An argument could be made that the redundancy lends the phrase a sense of hyperbole. And if that is what is meant, fine. Fine with this reservation, which is perhaps the larger sin in using this phrase: it violates Orwell’s advice from his essay Politics and the English Language (which ought to be read by any writer):
Never use a metaphor, simile, or other figure of speech which you are used to seeing in print.
Correct or not, this phrase is hackneyed and has been since before I learned to write. Or read. Or breathe.
Being a phrase that has been overused not only is a warning by itself, but also relates to my earlier allowance that it could be useful to lend emphasis to one’s point. The common-ness of the phrase removes the impact such redundancy might have given it. Compare he’s a complete idiot or he’s an utter idiot with he’s a complete and utter idiot. Either of the first two examples seem like more emphatic assertions of the person’s idiocy than the third.
Therefore, I declare this a complete and utter phrase to avoid.