Please update the StoryMap documentation that covers the "Allow duplication" setting and "Duplicate story" feature to explicitly note the latter is not supported with Public ArcGIS Online accounts.
For example, on the documentation page Publish a story or briefing, in the section discussing the "Allow duplication" setting, please add a note indicating that duplication of a story by a Public Account user is not supported. It currently states, "Use the Allow duplication check box to provide ArcGIS users with access to the story to duplicate the item", however, that is not true for Public accounts, which can access the story, but are unable to duplicate it.
