The function entity_translation_i18n_menu_form_menu_edit_item_validate checks if the path of the new item being created is not already in the database.
But this validation prevents to add more than 1 link with path equals to (option given by Special menu items module)
http://drupal.org/project/special_menu_items

Comments

Status:Active» Postponed (maintainer needs more info)

I don't understand your problem... That function should only prevent multiple menu links to the same node, i.e. it shouldn't affect menu links with the path set to <nolink> or <separator> at all. And either way, you should still be able add additional items via the menu configuration area.

Can you elaborate on what's not working as expected, and what the expected behavior is? Also, it would be helpful to have steps to reproduce the error on a clean install (with dev-versions).