Looks like unnecessary mapping of capability

Hello! Genode,

From time to time, I saw something like this:

l4_task_map(L4_BASE_TASK_CAP, L4_BASE_TASK_CAP, l4_obj_fpage(A.dst(), 0, L4_FPAGE_RWX), B.dst() | L4_ITEM_MAP);

I think it looks like a unnecessary mapping, because rather than mapping, we can just copy the capability like this, A = B.

Since I'm feeling like I miss something here, could you tell me the difference between two of them?

Best regards, Jaeyong