It appends all of the elements that you're adding to the list at the end of the list, so your assumption is correct, the result will be in the following order:
Note, however, that this order might not be preserved if you save it in a referenceset. I can only guarantee that it'll remain in this order during a microflow.
A list is a set in Mendix so adding the same objects will not help. So unless Payment1 and Payment1 are not the same Object nothing will happen. Furthermore because it is a set i don't think the order will be saved but i am not sure on that one.
I'm sorry for the confusing example. I'll edit it in my starting post.
the Payment column is just a simple string field. So all the objects in both lists are different objects.