#!/usr/bin/env perl

use strict;
use warnings;

my ($first, $second);
while(($first = <DATA>) && ($second = <DATA>)) {
    print "1: $first", "2: $second\n";
    if($first eq $second) {
        print STDERR "Duplicate line $first";
    }
}

__DATA__
Hello
World
Hallo
Welt
Servas
Woit
foo
foo