Amino acids are repeated in some sequences although the codons are different. How is this possible?
The reason is redundancy, which is an important characteristic of the genetic code. As said by the other person, many amino acids have more than one codon that will produce them. It is usually the second or third (especially) base of the codon that can vary without changing the amino acid. It is quite rare that the first base changes. There is one amino acid (methionine) which is coded for by only one codon. It is the beginning amino acid of every protein.
