This is something completely different. Your “duplicate content” is caused by the slider you are using. In every slide there is a div which is called background-parallax. These have an inline-style “margin: 10px” wich causes the offset. Your parallax container loads the same background image as in your slide.
Please check if you have some settings for the parallax-effect. Perhaps you could disable parallax as it does not seem to have any effect. In addition check if you can adjust the margin of your parallax-container. If all this won’t help you could add a style:
.background-parallax { margin-top: 0px !important; }
to your themes custom css (see above).
Another thing you should work on are the images itself. You are loading the complete images while only display a part of it. You should crop these images in order to load less data.
Best regards
-tim