MGNLUI-1292 you fix the same name siblings for dropping a node in to another. But the fix does not cover the dropping before and after a node. You should add the node name check in BaseDropConstraint for the other cases.
Furthermore it seems only to work on root level (see screenshot-1). Reason is the root node is not restricted to have same name siblings. There seems to be a setting for repository setup. See Repository.NODE_TYPE_MANAGEMENT_SAME_NAME_SIBLINGS_SUPPORTED