Shopping cart problem

Hi,

I am facing a strange problem with OC, when customers add products to the cart, it gets added twice instead of just once. I couldn't reproduce it for me but i am noticing a lot of abandoned orders and logging into the account shows the shopping cart to have 2 items (out of stock).

So i would like to know of a way to either completely disable the cart function and go to checkout directly OR use the cart system like in OC 1.4 where add to cart brought you to checkout page (with shopping cart functionality still there).

Also is there anyway to stop adding more quantity of a product to the shopping cart ?. My site sells downloadable content and so there's no need for a product to be ordered more than one in quantity.

Thanks

I already did the add to cart -> goes to shopping cart directly. However, what i am interested in now is how do i keep the quantity at 1 no matter how many times they click Add to Cart ?. I just need to know where the code for adding to cart is so i can put a quantity=1.

Help pplease

catalog/controller/checkout/cart.php