The duplicates tye mentions reside in the result set of a permutation effectivly operating on the (unique) index set of an array, rather than the (maybe) non-unique array members.

For an array qw(n o o n) List::Permutor will list all the 4! == 24 "solutions", of which only 6 are unique:

use strict;
use warnings;
use Algorithm::Loops qw(NextPermute);
use List::Permutor;

local $\ = $/;

my @x = sort qw(n o o n);    # sort required, c/f A::L docs
print "Algoritm::Loops::NextPermute says";
do {print "@x"} while NextPermute @x;

my $perm = new List::Permutor qw/n o o n/;
print "List::Permutor says";
while (my @x = $perm->next) {
    print "@x";
}
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Algoritm::Loops::NextPermute says
n n o o
n o n o
n o o n
o n n o
o n o n
o o n n
List::Permutor says
n o o n
n o n o
n o o n
n o n o
n n o o
n n o o
o n o n
o n n o
o o n n
o o n n
o n n o
o n o n
o n o n
o n n o
o o n n
o o n n
o n n o
o n o n
n n o o
n n o o
n o n o
n o o n
n o n o
n o o n
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