handling of duplicate files/images

Currently there is nothing to prevent the user from creating a duplicate image node. Since images and files have a unique ID that has nothing to do with the file as it exists in the directory structure, this creates a nightmare for duplicates since the image files end up with multiple entries in the image/node/file DB tables.

I think the most sane solution would be to treat a duplicate image like a node 'update' and not allow true duplicates to be created. Image is overwritten and new thumbnails are created. Or maybe old will be deleted and re-added...