Does protein consist of repeating units of amino acids?
They are repeating units in the sense that all of the units are amino acids. The R group is different in different amino acids (it is not just different theoretically). The different R groups contribute to the different shape, structure, function of proteins, and if all were the same, then we would have a very limited range of proteins.You will still say it is composed of repeating units, since all units are amino acids, and there are no foreign groups between them. 

Similarly, all repeating units of DNA/RNA are nucleotides, even if they aren't the same nucleotides.