Let's say that you'd like to create a nested menu in which one of the children links to the same path as its parent. Something like:
This works in theory-- the menu items will take you to the correct page. But if you're relying on Drupal to correctly set the active trail and expand the correct sub menus, you'll have to use a bit of a workaround. You're basically going to create dummy nodes that will redirect to the desired page rather than creating two menu links that have the exact same path. The parent (in menu) will redirect to the child's node.