Duplicated Items in Cart on Registration or Abandoned Carts

Hello all!,
Using OP 1.5.5.1, I have noticed that you can add the same product more than once in the cart. Is there a way to stop duplicate items from being added to cart?

I looked into this further, Here is the scenario,

Customer adds items to the cart,

Proceeds to checkout,

Goes through all the steps and accepts terms and conditions and clicks continue,

Reviews order and determines that they can spend more, then directly leaves the checkout process for Home or a Category, eithier by using the back button on the browser or a direct link.

Adds another item to the cart and proceeds to checkout,

Again goes through all the steps and clicks on continue,

Verifies the carts contents and totals, (at this point there is no indication in the cart that the items previous have been doubled)

Enters credit card info and clicks confirm.

Customer is taken to the success page.

Sounds normal right! Wrong, Looking at the sale (Sales>>Order), the items have been doubled from the previous checkout process. Mind you the customer did not enter any credit card information but did leave directly from a link or by using the back button. Either way the items get doubled. They do not get charged for the doubled items. But it is very annoying to see the invoice having double items.

With this clarification, can someone help with fixing this?

We're also having the same problem here, when staff are placing orders 1 after the other, normally using Cheque payment. Orders seem fine, but on the invoice, it includes double of everything.. product, shipping, tax etc. But the total is correct. Did you get any further understanding it? I have said for now to them to close the browser window after each order, to see if this will help. They don't want to clear their history after each order