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.
That is the case, the server doesnt log you out completely. I got that error message once, then i created a new account(nobody knew it) and tested some stuff about that, I closed AOL immediatly (with task manager) and then logged back on, and AIm gave me that message. No need to worry. Except if it happens every time.