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.