Glycine is the simplest amino acid, and it the only one that lacks a chiral center on the alpha carbon. Of the four groups attached in a tetrahedral arrangement to the alpha carbon, two are hydrogen atoms. In all other amino acids, one of the two hydrogens of glycines is replaced by a distinct functional group. The really interesting thing about biologically used amino acids is that it is always the same hydrogen of the two that is replaced -- all the 19 non-glycine amino acids are so-called "L-stereoisomers." Discovery of any one of the 19 amino acids other than glycine in a comet would be quite a story, and it would be even more surprising if there were a mixture of "L" and "D" stereoisomers other than 50/50.
My bet is that if another amino acid is found in cometary debris, it is asparagine, since it can form by the reaction 2*glycine - water.