Hmmm....
I don't know. I assume it must have happened at some point after you did some importing?
Have you done any monkeying with the DB by hand?
Here's what I would do if you can pull it off -- if ET Helper will let you do it, I would export all of your custom stuff into a file, copy out your .mon and .rac and .chr files and anything else custom you might have done, and then either restore the DB from a backup (I backup religiously with Davin's handy tool) or do a re-install. Then use ET Helper to re-import your custom stuff and put your mon, rac and chr files back.