This was initially reported in pydev thread "ttk.Treeview.insert() does not allow to insert item with iid=0" on 2018/3/16.  Igor, you should have mentioned on the thread that you had opened a tracker issue in response.  It was only happenstance that I did not open a duplicate.

I suggested there that 'if iid:' be replaced by 'if iid is not None:' but wondered if passing '"-id" ""' in the tk.call would be a problem.  If so, that would be a reason for the expanded check and suggest 'if iid in (None, ''):" as the replacement.

MRAB responded that '' is the id of the root of the tree.  But that does not tell me if passing a duplicate iid or that particular duplicate is a problem or not.