use strict;
use warnings;

my (@sets, $n, %c, @u, @a);
while (<DATA>) {
    chomp; push @sets, [split /,/];
}
for (@sets) { $c{$_}++ for (@$_); }
$n = $#sets + 1;
for (@sets) {
    print '[' . join(',', @$_) . '] = [';
    @u = (); @a = ();
    for (@$_) {
        if ($c{$_} != $n) { push(@u, [$_, $c{$_}]); }
        else { push (@a, $_); }        
    }
    for ((sort {@$a[1] cmp @$b[1]} @u)[0..1]) {
        print @$_[0] . ',';
    }
    print $a[0] . "]\n";
}

__DATA__
1,2,3,4,5,6
3,4,5,7,8
3,6,8,9

##</code><code>##

for (@sets) { $c{$_}++ for (@$_); }