Often allowed: a page slug must be unique among its siblings only, a post's within its blog, a FAQ page's under its parent; so two "about" pages under different parents are fine and reachable by their full paths. Not allowed: FAQ questions share one namespace across the account, custom entity records one per definition across all sites, categories and tags one per site. The generator appends -1, -2 on a collision; a custom URL is unique per site and is the usual cause of a real clash.