should have been more specific -- one way to enter duplicate trusted senders would be to add one, then refresh the resulting page (which would add another). I'll look at adding code that prevents duplicates from going in -- there should be no harm done, though, since more than one will mean the same thing as one when they're being checked.
The only way those can be added is through the website -- there's no code in the mail handler to add them, so I wouldn't think they were being added somehow by incoming mail. I suppose there are possibilities I'm not considering here, though