Hey Wibble,
In the following example I created another duplicate pair for show. The code looks at it as if the duplicates are not together, in otherwords I am assuming this could happen.

my $string = (join(".", sort split(/\./, "Pugh.Barney.Pugh.Barney.McGr
+ew.Cuthbert.Dibble.Grub")));
my @array = ($string =~ m/(\S+)\.\1/g); # Capture duplicate
$string =~ s/(\S+)\.\1//g;    # Eliminate duplicates
$string =~ s/\./\n/g;         # Change \. to \n
foreach (@array) {            # Print duplicates
    print "\n$_.$_";
}
print "$string";              # Print the rest of the list
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If you need to keep the string together you can use .= in the foreach loop and add ' around the dups as well.
The following will add single quotes around the dups.

    $string =~ s/(\S+)\.\1/'$1.$1'/g;    # Add quotes

and plugged into merlyn's reply

$_ = join(".", sort split(/\./, "Pugh.Barney.Pugh.Barney.McGrew.Cuthbe
+rt.Dibble.Grub"));
$_ =~ s/(\S+)\.\1/'$1.$1'/g;      # Add quotes
my @keepers = grep defined $_, /'(.*?)'|([^.]+)/g;
print map "$_\n", @keepers;
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Gyro