<?php

$firstname = array ( "John", "Tom", "Ben", "John", "David", "Julie", "David");
$lastname = array ("Kennedy", "Hyde", "Hughes", "Harper", "Walter", "Weber", "Walter");
$fullname = array();
$duplicates = array();
for ($i =0; $i < count($firstname); $i++){
	$fullname_tmp = $firstname[$i]." ".$lastname[$i];
	if (in_array($fullname_tmp, $fullname)){
		if (in_array($duplicates, $fullname_tmp)){
			$duplicates[] = $fullname_tmp;
		}
		else{
			$duplicates[] = $fullname_tmp;
			$duplicates[] = $fullname_tmp;
		}
		
	}
	else{
		$fullname[] = $fullname_tmp;
	}
}

echo "<pre>\n duplicates \n";

print_r($duplicates);

echo "\n full names \n";

print_r($fullname);