This happening to more and more often. I play PVE...I get into PvP (68th in line) and my first 5 minutes Error 304 (This Account is Already Logged In).
Error 304 typically occurs after you have been booted from the ESO megaserver and are attempting to log back in. When this occurs, it can sometimes take a bit of time for the server to register that your character is no longer in game, so it won't allow you to log back in right away.
If you encounter this error, you should wait a full 15-20 minutes before attempting to log in again.