#!/usr/bin/perl

use strict;
use warnings;

my %seen;

open my $out, '>', 'output.txt' or die $!;
while( my $line = <DATA> ){
    my ($id) = split /\s+/, $line;
    unless( $seen{$id}++ ){
        print $out $line;
    }
}
close $out or die $!;

__DATA__
5.0           2.08      9.496     23
10.0           2.08      5.917     39
10.0           2.08      5.917     39
15.0           1.02      6.823     75
20.0           1.01      14.810     80
20.0           1.01      14.810     80
25.0           1.61      27.117     37
30.0           1.61      71.563     72
30.0           1.61      71.563     72