shopping cart session issue

Question

my shopping cart adds items to cart from different systems. can any one help me on this. 

Other users cart which is added from other system is visible in my system. 

Here is my shopping cart method

 

static ShoppingCart() {

if (HttpContext.Current.Session["ASPNETShoppingCart"] == null) {

Instance = new ShoppingCart();

Instance.Items = new List<CartItem>();

HttpContext.Current.Session["ASPNETShoppingCart"] = Instance;

} else {

Instance = (ShoppingCart)HttpContext.Current.Session["ASPNETShoppingCart"];

}

}