ShoppingCartValidator incorrect validation when cart is set to AddProductsToBasketInSinglePositions

[Algorithman]

Describe the bug
Checkout is validating quantity wrong for Products which are set to ManageInventoryMethod.ManageStock, StockQuantity=1, BackorderMode.NoBackOrders (with cart set to AddProductsToBasketInSinglePositions).

It gets the quantityToValidate here but then adds the quantity again here which then adds the insufficient stock warning.

Why is the quantity added again to quantityToValidate? Shouldn't quantityToValidate just be set to the quantity of the sum?

To Reproduce
Steps to reproduce the behavior:
Create a product with these settings: ManageInventoryMethod.ManageStock, StockQuantity=1, BackorderMode.NoBackOrders.
Set the cart to single position adding.
Put the product into the shopping cart and try going to checkout.

Desktop (please complete the following information):

• OS: Win10x64, Linux x64
• Browser Chrome

[Algorithman]

PS: please add also to 5.0.5.x

[Algorithman]

I think the main problem here is that you don't check if the cartItem is already in the cart or not.
Extending the query to

quantityToValidate += cartItems
    .Select(x => x.Item)
    .Where(x => x.ProductId == p.Id && x.ParentItemId == null && x.Id != cartItem.Id)
    .Sum(x => x.Quantity);

should do the trick.
It adds the quantity if ValidateProductAsync is actually called to add a product and does not if it's called just to verify the cart.
I don't know if this also has to be done for the combination products (since we don't deal with them), but my guess is yes.