Hello Cryssal,
I am not sure if this is the case but when the default cost assignment and deviating cost assignment is same, Summary will show the same cost assignment twice.
i.e
Default Cost assignment for the ER is : cost center X
and for one particular expense in the expense report, you maintain Deviating cost assignment as cost center X only and for remaining expense in the same expense report, you don't maintain anything under deviating cost assignment.
In such case its shown twice on the overview or summary. But when you edit or view the expense report, its displayed correctly.
I am not sure if this is true for Cloud for customer also. But I have seen this behavior in by design systems.
Hope this helps. 
Regards,
Harshal