MolliePlugin >= v2.2.0 may duplicates order items resulting in higher charges

[kyunal]

Describe the bug
The updated MolliePlugin that re-uses existing mollie orders rather than creating new ones can cause erroneous charges by not re-using or cancelling order lines correctly. While beneficial to the shop owner unless the customer notices, I do not think this is intentional.

To Reproduce
Steps to reproduce the behavior:

1. Go to mollie checkout (in our storefront: AddingItems -> ArrangingPayment, then create intent)
2. Return from mollie checkout again (in our storefront: ArrangingPayment -> AddingItems)
3. Go to mollie checkout again (in our storefront: AddingItems -> ArrangingPayment, then create intent)
4. See error

Expected behavior
Only order line items present in the vendure order end up being present in the mollie order.

Environment (please complete the following information):

• @vendure/core version: 2.2.0
• Nodejs version 18
• Database (mysql/postgres etc): pg14.9.0

Additional context
I haven't debugged the issue but I believe it might stem from here https://github.com/vendure-ecommerce/vendure/blob/7d50b5446d823f933103442ac6ce99f5bd02e5b6/packages/payments-plugin/src/mollie/mollie.service.ts#L486C13-L492C14

If the order lines are equal, the first condition is not met and they will be pushed again into the mollie order by using the add operation, while old order lines are still present. I think for safety reasons it might make sense to simply cancel all old lines and then create new ones rather than to mess with re-using the existing lines, but that is just my gut feeling.

[michaelbromley]

Paging @martijnvdbrug - when you get time can you share your opinion on this? You're now much more familiar with this plugin than I am :)