Determination of one amino acid by more than one codon is due to

Degeneracy of codons is the redundancy of the genetic code. A single amino acid may be specified by  more than one codon i.e., called degeneracy. Degeneracy is due to the last base in codon (which is known as wobble base). Thus, first two codon are more important to determine the amino acid and third one differ without affecting the coding i.e., known wobble hypothesis, proposed by Crick which establishes an economy of tRNA molecule.