Cart with Mysql - PHP

Hi,

I have a question about my shopping cart. The cart and session variable are stored in mysql database. I have the following code structure:

//basically no html in these include files. Purely GET -POST stuff //and mysql queries //etc.
if(isset($_POST['add'])){
include('addtocart.php'); 
include('viewcart.php'); 

}
elseif(isset($_POST['update'])){
include('updatecart.php'); 
include('viewcart.php'); 
}
elseif(isset($_GET['pid'])){//delete item
$pid=$_GET['pid'];
include('deletecart.php'); 
include('viewcart.php'); 
}
else{
include('viewcart.php'); 
}
.
.
.
//html and cart content is displayed...  

PHP:

My question is: Whenever I refresh the page whilst there are items in the cart, the quantity gets doubled. How can I prevent that? (I believe the code directly executes the following part:

if(isset($_POST['add'])){
include('addtocart.php'); 
include('viewcart.php'); 
}  

PHP:

 

after adding an item to the cart, redirect the user the the page you want... in order to loose the post data