When trying to update crouton I kept getting the following error message:

Preparing to unpack .../linux-image-3.8.11_20150314_amd64.deb ...
Ok, aborting, since modules for this image already exist.
dpkg: error processing archive /var/cache/apt/archives/linux-image-3.8.11_20150314_amd64.deb (--unpack):

After some digging I found this thread which explains how to fix it. The source of the problem was the fact that I had installed a modified kernel to allow VirtualBox to work properly. The way to update crouton is to remove the repository for that kernel

rm /etc/apt/sources.list.d/crouton-packages.list

After removing that repository the update completed successfully.