i just sort of fixed @elBranfords problem inside the walker. don’t know how “nice” this fix is, but it works for now with my site. the fix is inside the start_el function, so there is no change in anything else.
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it checks if the current item has the same id as the parent item and than skips it. i set the parent id to 0 (no parent id), so it only works with two level navigations. maybe someone finds a way to find the proper parent id (i didn’t really look for one). then it would work for navigations with more then two levels.