Sub-menu page that mirrors main page?

Sorry if this has been asked before, I ran a search but couldn't find anything.

I have a main menu on my site, and some of the pages on the menu have a sub-menu. What I'm trying to achieve: when someone clicks on the first item in the sub-menu, I want them to see the same content they would see if they clicked on the main menu item. In the screenshot below, if someone clicks on Mountains or Sherwin Lakes, I want them to be taken to the same page. The second page and subsequent pages in the sub-menu lead to different pages which is fine, but I want the main menu and the first sub-menu item to be the same.

The solution I'm using right now is rather clumsy and time-consuming: any time I need to update the main/first page, I update the main one, duplicate it, and then make the duplicated page the first subpage in the menu.

I'm thinking there must be a better solution for this, where the first sub-page automatically mirrors the main page.

Thanks for any advice!