My navbar behaves weirdly. When I’m on this page on tablet or smaller (thanks to the web inspector), then I open the burger menu, then I close it and finally I resize my window in order to view the desktop version, the mobile menu links are visible in addition to the desktop links in the navbar:
This setup is unique and appears to be more complex than it needs to be.
The scenario you’ve described seems unlikely to occur in the real world. Not sure when a user will have the browser open at desktop width, change to a skinnier width, open the menu, then go back to desktop? I guess it’s possible but not sure why someone would ever do that.
Attached a screen grab showing a quick hack. You can leave the structure as-is but create duplicates of the highlighted styles and ensure each is set to display=none on the desktop. I tested this and the issue goes away.