Menu items are being shown twice on mobile

Struck is designed with two (2) menus on the desktop view. The top menu and the slideout menu are two different menu locations. That gives you the flexibility to have a lot of menu items on your site without overwhelming the user. When the view becomes too small for the top menu, the top menu items are automatically added to the slide out menu location. That way, mobile users and desktop users have the same menu options.

If you want to only show the slide out menu on mobile devices, you can do this with custom css. Navigate to Theme Options > Customize and add this to the custom css box:

.tw-menu-icon {
    display: none;
}
@media only screen and (max-width: 991px) {
    .tw-menu-icon {
        display: block;
    }
}

This will hide the menu icon on desktop, but show it on mobile devices. If you want the slide out menu to have the exact same items as the top menu on mobile, you'll want to add a blank menu to the slide out menu location (create a new menu, but add no items). The top menu is automatically added on mobile devices.