%Image::ExifTool::UserDefined = (
    'Image::ExifTool::Composite' => {
        UniqueKeywords => {
            Require => 'Keywords',
            ValueConv => q{
                my @list = ref $val eq 'ARRAY' ? @$val : $val;
                my ($item, %found, @rtn);
                foreach $item (@list) {
                    $item =~ s/^\s+//;   # remove leading white space
                    push @rtn, $item unless $found{$item};
                    $found{$item} = 1;
                }
                return \@rtn;
            },
        },
    },
);