OpenCart set only one item in cart.

If you want to display only one item in cart and the add more OR update cart quantity then follow below step:

1. First edit System / library / cart.php file

2. Find below code:
$this->session->data['cart'][$key] += (int)$qty;

3. Replace with this one:
$this->session->data['cart'][$key] = (int)$qty;

So its not update cart quantity.

Enjoy!