The author information you enter during submission isn't actually explicitly linked to users in the system -- that's a feature, not a bug.
In a nutshell, the author information you enter during submission is actually submission *metadata* -- it's tied to the article and can be searched against etc., but is not tied to users within the system. This allows users who are Authors to submit files on behalf of real-life authors, for example.
This does become an issue when small errors are introduced or different emails are used and it looks like the same author appears twice in the Browse by Author list, for example, but it's unavoidable -- we have to make allowances for the possibility of different authors having the same name, for one thing. My advice would be to check the submission metadata for each 'unique' author's articles, and ensure that their author information is exactly the same, including spaces, etc.