Unique permutations

unique_permutations.pl

# Generate all unique permutations of an array
# with possibly duplicate elements (multiset).
# unique_permutations(('A', 'B', 'B')) -> ABB BAB BBA
sub unique_permutations
{
  my @results;
  my (@list) = @_;

  if ($#list == -1) {
    return [()];
  }
  else {
    # Create a hash with the unique elements of the list as keys and the index 
    # of these elements in the list as values.
    my %index_hash;
    @index_hash{@list} = (0..$#list);

    # Loop over the unique elements of the list.
    my ($unique_element, $index);
    while (($unique_element, $index) = each(%index_hash)) {
      # Create a sublist of the list excluding the unique element.
      my @sublist = @list;
      splice(@sublist, $index, 1);

      # Loop over all the permutations of the sublist (recursively).
      for my $subpermutations (unique_permutations(@sublist)) {
        # Append the unique element to this subpermutation and add the result 
        # to the final list of results.
        push @results, [(@$subpermutations, $unique_element)];
      }
    }
    return @results;
  }
}

unique_permutations.py

def unique_permutations(iterable):
    # Generate all unique permutations of an iterable
    # with possibly duplicate elements (multiset).
    # unique_permutations('ABB') -> ABB BAB BBA
    if not iterable:
        yield []
    else:
        xs = list(iterable)
        for x in set(xs):
            i = xs.index(x)    
            for result in unique_permutations(xs[:i] + xs[i + 1:]):
                yield result + [x]

Inspiration: http://mail.python.org/pipermail/python-list/2009-January/1189787.html