$ perl -le'
use List::Util qw/ shuffle /;
my @chars = qw/ A B C D E F /;
my $length = 12;
my $repeats = 2;

my @set = shuffle( ( @chars ) x $repeats );
die "Error!\n" if @set < $length;

print join "", @set[ 0 .. $length - 1 ];
'
FCBECFADABED