dear all. i have tried to prevent the duplicate data at my project. but until now it still make duplicate. i try this code but still not work:

  $cek_user= "SELECT Model, Serial_number, Line FROM inspection_report WHERE Model='".$Model."' AND Serial_number='".$Serial_number."' AND Line='".$Line."'";
  $cek_data=mysql_num_rows($cek_user);
  if($cek_data!=0){
                    echo "Data already exists!";
                   }
  else{
  $sql = "INSERT INTO inspection_report ";
  $sql.= "(Model,       Serial_number,       Line,        Shift,     Inspection_datetime,         Range_sampling,       Packing, ";
  $sql.= "Accesories,       Appearance,      Tuner,        General_operation,       Remark, ";
  $sql.= "NIK) ";
  $sql.= "VALUES ('";
  $sql.= $Model."','".$Serial_number."', '".$Line."','".$Shift."','".postVar('insp_date')." ".postVar('time')."','".$Range_sampling."','".$Packing."','";
  $sql.= $Accesories."','".$Appearance."','".$Tuner."','".$General_operation."','".$Remark."','";
  $sql.= $NIK."')";
//echo $sql;

$result=mysql_query($sql) or die(_ERROR26.": ".mysql_error());
//echo $result;
}
mysql_close($dbc);
}

ancak yine de çalışmıyorsa, yardım lütfen.