This solution preserves order, if that is an issue.

johngg@aleatico:~/perl/Monks$ perl -Mstrict -Mwarnings -E 'say q{};
open my $inFH, q{<}, \ <<__EOD__ or die $!;
ID1<TAB>nick-john-helena
ID2<TAB>george-andreas-lisa-anna-matthew-andreas-lisa
ID3<TAB>olivia-niels-peter-lars-niels-lars-olivia-olivia
__EOD__

while ( <$inFH> )
{
    chomp;
    my( $pre, $post ) = split m{(?<=>)};
    say $pre, join q{-},
       do { my %seen; grep { ! $seen{ $_ } ++ } split m{\s?-\s?}, $pos
+t }
}'

ID1<TAB>nick-john-helena
ID2<TAB>george-andreas-lisa-anna-matthew
ID3<TAB>olivia-niels-peter-lars

[download]

I hope this is of interest.

Cheers,

JohnGG

In reply to Re: How can I make a string unique (quicker than my approach at least) by johngg
in thread How can I make a string unique (quicker than my approach at least) by Anonymous Monk

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