while (<DATA>) {
  print /(\d+)/ ? ", $1" : $s{$_}++ ? '' : $a++ ? "\n$_" : $_ for split /\s*,\s*/;
}
print "\n";
__DATA__
a, 1
aa, 3
aa, 5
aa, 8
b, 2
b, 3
bb, 5