Inserting data into sql it inserts two into rows, and i dont know why

When Inserting data into SQL it inserts two into rows, and I don’t know why.

most probably of the if statement I added after the results you can find it as my comment:

// id_no update

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And I used the select query two times to fetch the id and make it an auto-increment thing.

my table:

<?PHP
  $query = "SELECT id_no FROM db_name ORDER BY id_no DESC";
  $result = mysqli_query($con,$query);
  $row = mysqli_fetch_array($result);
  $lastid = $row['id_no'];
  if(empty($lastid))
  {
    $number = "SX000001";
  }
  else
  {
    $idd = str_replace("SX", "", $lastid);
    $id = str_pad($idd + 1, 6, 0, STR_PAD_LEFT);
    $number = 'SX'.$id;
  }
?> 
<?PHP
  if(isset($_POST['add_id']))
  {
    $id_no = mysqli_real_escape_string($con,$_POST['id_no']);
    $sql="INSERT INTO `db_name`(`id_no`) VALUES ('$id_no')";
    $result=mysqli_query($con,$sql);

    // id_no update
    if(mysqli_query($con,$sql))
    {
      $query = "SELECT id_no FROM db_name ORDER BY id_no DESC";
      $result = mysqli_query($con,$query);
      $row = mysqli_fetch_array($result);
      $lastid = $row['id_no'];

      if(empty($lastid))
      {
        $number = "SX000001";
      }
      else
      {
        $idd = str_replace("SX", "", $lastid);
        $id = str_pad($idd + 1, 6, 0, STR_PAD_LEFT);
        $number = 'SX'.$id;
      }

    }
    else
    {
        echo "Record Faileddd";
    }
    
    
    if($result)
    {
      $success="Post has been added successfully";
    } else 
    {
      $error="Something went wrong!";   
    }
      $id_no = '';
    }
?>

Please don’t downvote this question I asked something wrong or a similar question is already available, just comment me I will delete it

>Solution :

You should check if $result is truthy to see if the insertion succeded (without running another query):

$id_no = mysqli_real_escape_string($con,$_POST['id_no']);
$sql="INSERT INTO `db_name`(`id_no`) VALUES ('$id_no')";
$result=mysqli_query($con,$sql);

// id_no update
if($result)
{
  ...
}