That does seem kind of buggy - since the first three i can understand - usernames are different. but if you isolate just those last two, they look identical (at least in terms of the fields shown there). You may want to open a support ticket if you are under current maintenance.
If you think that your users are all "in" the system in one way or another already, you could disable the setting that allows email tickets to create new accounts. (Setup -> E-Mail -> Options -> "create accounts for unrecognized senders")
I have seen the same problem coming from two sources. One is the person is using something like Incognito and the system does not recognize them even if they are an existing client. I have seen the system create over 18 accounts for s single user due to this. If the person is a member of Google Groups also it could cause this issue. Do you have "Client Can Create Account" set to "Only if client's email matches an accepted domain" on both the client and email options?