There is absolutely no constraint about labels in KnpMenu. But while working on #45, I figured that the label is used as key in the return value of getBreadcrumbsArray, meaning the method is totally broken when a label is used twice in a path in the tree.
Fixing it would require a BC break as the bug comes from the design chosen for the method (which still comes from ioMenuPlugin)
Refactored the format of the breadcrumb array
Missing namespace in MenuFactory default item class fix #47