The triplet nature of the genetic code make for a possible 64 codons: 61 code for amino acids present in most polypeptide chains while the remaining 3 are special-sense codons, since they only signal for the termination of the polypeptide chain.
Thus, the pattern of the genetic code permits that a base appear more than once in a codon. For instance Adenine occurs twice in both AAU and AAC codons, which code for asparagine.
