Overwrites existing thumbnail files

Issue Summary

I'm using image_import with image and image_exact on Drupal 4.7. When importing images, the module seems to correctly notice the presence of existing image files with the same name and adds _0 to the end of the file name. However, the _0 isn't added to the end of the thumbnail filename. I'm not sure exactly which module is causing this problem, but it doesn't seem to happen when uploading two images with the same name through the standard creation of image nodes.