Make a new boolean and add it to the char file, and when the player logs in, check if it's true or not.
Name the boolean "player_logins" or something, and whenever they login, add +1 to it. If they have 1 login, then make it send the message.
I'm not a java coder, so this probably isn't the best way to do it. Just ask one of the 'developers' here...