Has anyone seen the movie "The Lizzie McGuire Movie?" I've a question about that ...?

If you look up "The Lizzie McGuire Movie Full cast and Crew" on Google, they give you every actor/actress's name(s). It'll say that Lizzie and Isabella are BOTH played by Hilary Duff. How is this accomplished, seeing how they sang together at the end in "This is What Dreams Are Made Of"?