Short version: It’s a bug in the theme. Until the developer fixes it, a temporary fix would be to rename sidebar-footer.php to sidebar.php.
Long version: What’s happening is that on your 404 page, your theme calls get_search_form(). That creates the first search form. Then your theme calls get_sidebar(), except that your theme doesn’t contain a sidebar.php, it only contains a sidebar-footer.php. So WordPress calls in the default sidebar.php, which also calls get_search_form(), leading to duplicate search bars.