When "Show parent in nav dropdown" is checked, the parent page is included when you hover over and display the dropdown menu to access subpages (also known as child pages). So, you will see the top page twice in the drop down.
In the image below, a parent page, 'FAQ,' is included in the dropdown menu.
Change nav dropdown settings
To change the "Show parent in nav dropdown" setting, navigate to your global site settings from Website > [Name of Your Site] > Site settings > Basic.
If you uncheck "Show parent in nav dropdown," and click Save site, the top navigation dropdown will display without the parent.
Other items that can be changed in the global settings area include the name of the site, who the primary broadcaster for your site is, and additional settings that can be turned off across the entire site.