Checkout TotalPrice is counting shipping price twice

[GrzegorzKrauze]

Using shipping broker as a delivery method source.

Scenario:

1. not logged in user is adding one (or more) product into the new cart -> a new guest cart is created with this product as a item

2. user goes to the cart, and goes to the avarda checkout -> user can see shipping methods. There is also a items call:
   https://stage.checkout-api.avarda.com/api/partner/payments/{purchaseid}/items . It adds the selected product as a item in given purchaseId in Avarda

3. update request is called -> Avarda API is sending request for update and it receives the correct amount in TotalPrice, which is "subtotal" + "selected shipping method price"

4. User refreshes the checkout -> The items are sent again to avarda but this time the qoute shipping is added as a item.

5. In result on the update request, the selected shipping price is added twice.

When I patched the code not to add a shipping as a item in the quote then it is working as expected:
-> when the shipping is already chosen and the user is refreshing the page - the amount is not increased by the additional shipping cost
-> when placing the order the "shipping item" is added.

My patch only commented this line as a workaround:

Referring this line
https://github.com/avarda-ab/Magento2-Checkout-module/blame/3c99643f1b20a053c7240ac8b91cab1416dfaff0/Plugin/Model/Quote/QuoteCollectTotalsPrepareItems.php#L179

Is it necessary to add the "shipping item" from the cart shipping address in any case?

[GrzegorzKrauze]

In other words the difference is on enter the checkout page, when the module is sending

PUT https://stage.checkout-api.avarda.com/api/partner/payments/{purchaseid}/items

1. If user is refreshing (or getting back to) the checkout page - the "shipping item" is added here
2. If the user is entering the checkout page the first time - only the product items are added to avarda with this PUT request

after the "1 point" scenario the "update" request to avarda is adding a selected shipping on top of already added items. So if you added one product and entered the checkout module twice you have 3 items in avarda purchase, from which 2 are shippings.
after the "2 point" scenario the "upfate" request is also adding a shipping method item but there is no extra item here.

Of course this is the case on the shipping broker variant.
I haven't had occasion to test it with the regular magento-defined delivery methods.

[GrzegorzKrauze]

I've moved it to the correct module version:
avarda-ab/Magento-2-Checkout-3-module#48