I suggest changing your password again - yes I know its very frustrating to constantly change you password but such is life, "Paranoia makes my life easier and the hackers life harder"
When you logged in and it gave the "user already logged in" error message, had you just logged in and then out again? Becuase if thats the case then it could very well be that AIM hadn't registered that you had logged out and therefore thought that you're a second person logging in.