use strict;
use warnings;

use Test::More tests => 1;

my @in = split (/\n/, <<EOT);
aaa Foo: 1234 xyz
bbb Foo: 1234 yzx
ccc Foo: 1234 yxz
ddd Foo: 2345 zyx
EOT

my @want = split (/\n/, <<EOT); 
aaa Foo: 1234 xyz
ddd Foo: 2345 zyx
EOT

my $number;
my $index;
my %seen;
my @out;

for my $line (@in) {
    if ($line =~ /Foo: ([\d]{4})/) {
        $number = $1;
        $index  = index ($number, $line);
    } else {
        $number = '';
        $index = -1;
    }

    push @out, $line unless $number ne '' && $seen{$number}++ && index ($number, $line) == $index;
}

is_deeply (\@out, \@want);