Degenerate and redundant mean the same thing in this context.
Selenocysteine is an unusual amino acid. I have an older copy of Voet and Voet's textbook which discusses it with respect to E. coli
. Its tRNA bears the anticodon UCA, which pairs with the codon UGA (which is ordinarily the Opal stop codon). Therefore, I would say that this is a partial exception to the rule that the genetic code is unambiguous. I used the word "partial" because the alternatives are an amino acid and a stop codon, not two amino acids. Could there be another amino acid like selenocysteine? Yes, but I am not aware of any evidence that there is one. Here is a discussion of whether or not selenocysteine should be called the 21st amino acid: http://jnci.oxfordjournals.org/content/96/7/504.full
There are a few other examples of stop codons which do not always mean stop. In ciliated protozoa UAA and UAG are translated as Gln. However from Voet and Voet's treatment (which makes it sound as if there is only one true stop codon in these organisms), it sounds as if this example illustrates that the genetic code is not quite universal, not an exception to the rule with respect to its being unambiguous.