Webpage with dropdown navigation

My first attempt to make dropdown navigation without using Bootstrap. I tried to reuse the html code for navigation for popup menu (for smaller devices) but I didn't manage to figure out how.. so I had to rewrite all again for popup and ended up with a lot of duplicated code for both html and CSS.. Any suggestions to make it better will be appreciated! And in the popup, the nav item after the dropdown should move down and it should be seen when the dropdown is open but I don't know how to make that happen. Can anyone help me with this issue please?? Thank you:)