How to prevent image files being duplicated while importing nodes with the Feeds module?
I found a solution to this. What you need to do is to install the Media Feeds module.  

In your CSV file there is a column called e.g. image and reference the image using public://myimage.png.

Then in your mapping delete the original image mapping and create a new image mapping. When you do this you will have a new option for the image mapping.  Make sure you set the target configuration to "Source is exactly the filename".  

No more duplicated images.