วิธีเช็คไม่ให้เพิ่มข้อมูลซ้ำลงในตารางดาต้าเบส ควรใช้วิธีไหนดีคะ

<?php
$host		=	"localhost";
$username_db	=	"root";
$password_db	=	"crmsosys456";
$db	= "crm"; // database name
$databases = mysql_connect($host,$username_db,$password_db) or die ("can't connect database");
//mysql_query("SET NAMES UTF8");
mysql_select_db($db) or die("Can't select database");	


$result = mysql_query("SELECT email_address FROM leads");

while($row = mysql_fetch_array($result))
  {
	  if ($row['email_address'] != "")
	  {

  //echo $row['email_address'];
  //echo "<br />";

  $email_address = $row['email_address'];

  $email_address_caps = strtoupper($row['email_address']);

 //echo "$email_address_caps";
 //echo "$email_address";
 //echo "</br>";
	  
$sql = "INSERT INTO email_addresses(email_address,email_address_caps) 
VALUE('$email_address','$email_address_caps')";

mysql_query($sql) or die ("error=$sql ".mysql_error());
	
	}
	 
  }
////////////////////////////////////////////////////////////////////

$query = mysql_query("SELECT id FROM leads");

while($row2 = mysql_fetch_array($query))
{
	$id = $row2['id'];
	//echo "$id";
	//echo "</br>";

 $sql1 = "INSERT INTO email_addr_bean_rel(bean_id,bean_module,primary_add,reply_to_address) 
 VALUE('$id','Leads','1','0')";

 mysql_query($sql1) or die ("error=$sql ".mysql_error());

}
///////////////////////////////////////////////////


$query4 = mysql_query("select id FROM email_addresses");

while($id_mail = mysql_fetch_array($query4))
{
	$id_m = $id_mail['id'];

	//echo "$id_m";
	//echo "</br>";

$mailaddress =  mysql_query("SELECT email_address FROM email_addresses WHERE id='$id_m'");

$id_lead = mysql_query("SELECT id FROM leads WHERE email_address = '$mailaddress'");

$sql2 = "INSERT INTO email_addr_bean_rel(email_address_id) VALUE ('$id_m') WHERE bean_id ='$id_lead'";

mysql_query($sql2) or die ("error=$sql ".mysql_error());

}

echo "complete";
?>