Is there only one mRNA code for each amino acid?

No, there is not just one mRNA code for each amino acid. In the genetic code, multiple codons (three-nucleotide sequences) can specify the same amino acid, a phenomenon known as redundancy or degeneracy of the genetic code. For example, the amino acid leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This redundancy helps mitigate the effects of mutations in the DNA sequence.