here's what happens. When the page loads, a script looks for your navigation and creates a drop down menu. For each link in the original navigation, the script adds an option to the drop down.

The script looks for a "nav" element with "a" elements inside it. The problem is you have 2 menu made of nav elements with links in them.

The second one seems to be meant to be an overlay but it is hidden for some reason and I can't see any way to use it.

But the result is your script is getting the links twice and adding them twice to the drop down.