Menu in select list is duplicate

Hi, really helpfull plugin, thanks!
This is my website.
I have a problem, probably with theme, that duplicates the list of menu voices. My menu is a walker class menu. When I reduce the window of my browser a voice named “Go” (I think is about the theme) appears on the logo and the list is hide at the bottom of the search form. If I click I see the double menu.

Is strange because if I deactivate the plugin I don’t see any responsive menu.

Have you any suggest?

Thanks & regards.

https://www.ads-software.com/extend/plugins/responsive-select-menu/