DNA molecules consist of chemically linked sequences of the bases adenine, guanine, cytosine, and thymine, denoted A, G, C, and T. A sequence of three bases is called a codon. A base may appear more than once in a codon. How many different codons are there?

64 codons

Explanation:

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.