avoid dublcate entering number

Hi brothers
I need urgent help for my problem its about :
I am using following code to avoid my user to inter number used before its work perfect always except last a few days I found duplicate number in my data base table
part of code where process the number

PHP:

<?php
$message = $idn;
if (strlen($message) != 10){
   echo "number must 10 digit";
} 
else 
{
if (!$name | !$add | !$colour | !$yea | !$country | !$branch | !$price
 | !$idn) {
die('<a href="everyuser.php?tag=">sorry idn lese than 10</a>');
} 
if (!get_magic_quotes_gpc()) {
$idn = addslashes($idn);
     }
 $usercheck = $idn;
 $check = mysql_query("SELECT idn FROM rescar WHERE idn = '$usercheck'") 
 or die(mysql_error());
 $check2 = mysql_num_rows($check);
 
 if ($check2 != 0) {
         die(' sorry this number used before ( number &nbsp;&nbsp;'.$idn.'&nbsp;&nbsp;) ');
                 }
?> 

could any one help and tell me what happened exactly and how to fix this problem .
thank for your help