Problem: The main image in the one-page layout is showing un twice in a row. One image is responsive and right next to it is the same image again but not responsive. So you get the main image sliding over itself.
Solution: It is very possible that you have placed the Front Page inside the One Page Layout menu.
The Front Page (page with the ‘Layout: One Page’ template) works as a container for all the rest of the pages, which you put inside your one page layout, so you should never put it inside the Front Page (Acid One Page contents) menu. Simply navigate to Appearance > Menus and make sure that your Front Page is not placed inside the Front Page (One Page) menu.