That would not be a good thing, because in that case, that codon might sometimes code for one amino acid and sometimes for the other one. In other words, the protein will not be 100% the same every time. That would not be good for the function of that protein, because every amino acid is different (some are polar, some are hydrophobic, some are acid, some basic), so a particular protein must be made in the exact same way in order for it to work.
