Hi Thomas,
I am not sure, but I see a weird integrated mobile drop-down search form of some sort, that is not visible by default. I guess it is also loaded as the theme search bar and the plugin automatically overrides it. The mobile menu is then tries to make a duplicate of some sort, that probably causes the multiple outputs.
I can’t exactly tell by the screenshot though. Maybe adding the search shortcode manually would be the best idea, as the integrated search form seems to be strictly designed for this theme.