Actually, it is possible for all his screennames to be online at the same time. Its called multilogin where you can login more than one name at a time. Ok. I just tested this on myself. When you go to profiles you have the option to add a new public profile to one of your screennames. I added a profile to my screenname and it treats it like a completley different screenname with a seperate profile and a seperate name that has to be unique on yahoo and can't be duplicated. The only difference is that the person who uses another profile on their screenname, the buddylist is the same. But you, since you added both names, will see both the names come on at the same time and logoff at the same time.
I moved my profile name to different group lists, and it successfully moved it. But when I eraced the profile, and when I logged onto my sn, the profile name on my other list was not lit and when I tried to move it, it says does not exist. The only thing is that he could erace the profile but you have the name on your list and try to move it, but he already had deleted it. If he still talks to you on it, its possible that he could always go back and add the same profile again to his name as long as its not being used by someone else. Chances are he eraced it.
The only way I know that can test if a profile name or screenname are the same person is if they login or out at the same time. You'll just have to look real fast if you want to catch them all if he has alot. Keep in mind multilogin is also a possibility, but all names have to be existing And don't always login at the same time.
Multilogin just doesn't tell yahoo that a name is already logged in, so more than one can be online, but they have to be signed on individually, at least for mine.