Just for fun(?):

use strict;
use warnings;

my $wantedLines = shift;
my %seen;

while ( <> ) {
  print unless $seen{$_}++;
  last unless --$wantedLines;
}
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Update: Sorry, I posted the wrong version. Replace the contents of the while loop with:

  unless ( $seen{$_}++ ) {
    print;
    last unless --$wantedLines;
  }
[download]