Importing Extra Images

I am making my thumbnails larger by exporting my product list and reimporting it (after having changed the size at which the thumbnails are automatically calculated). This is working fine for the main product images but when I try doing this with any extra images I have for my products it is adding another copy of the images rather than overwriting what is already there. In other words, it is leaving the original copy of the extra image and adding another copy of the same image.

Is there any way of using the import feature to overwrite the extra images rather than having to go in and reload them manually or ending up with two of each image?

It seems strange that this is working fine for the main images and thumbnails but not for the extra images.

Hopefully someone can help me out here and so save me hours of work.