use strict;

my $prev;
while (<DATA>) {
  next if /^\s*$/;
  chomp (my ($curr, $val) = split /,\s+/);
  print $prev ? "\n" : "", $prev = $curr if $curr ne $prev;
  print ", $val";
}
print "\n";

__DATA__
a, 1
aa, 3
aa, 5
aa, 8
b, 2
b, 3
bb, 5