another solution

use strict;
use warnings;

<DATA>;     #throw away first line
my %names;

while (<DATA>)
{
   my ($name) = (split ' ',$_)[1];
   print unless $names{$name}++;
}

=PRINTS
123 john
11  peter
87  helen
=cut


__DATA__
id  name
123 john
34  john
567 john
11  peter
899 peter
87  helen
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