Quick and dirty. If the order of the names matter, this won't work. If there are duplicate ID tags, this won't work.

use warnings;
use strict;

my %seen;

while (my $line = <DATA>) {
    chomp $line;

    my ($id, $data) = split /\s+/, $line;

    next if ! $id || ! $data;

    $seen{$id}{$_}++ for split /-/, $data;
}

for my $id (sort keys %seen) {
    printf(
        "%s\t%s\n",
        $id,
        join '-',  keys %{ $seen{$id} }
    );
}

__DATA__
ID1    nick-john-helena
ID2   george-andreas-lisa-anna-matthew-andreas-lisa
ID3   olivia-niels-peter-lars-niels-lars-olivia-olivia
[download]

Output:

ID1    helena-nick-john
ID2    george-lisa-anna-matthew-andreas
ID3    niels-peter-lars-olivia
[download]