Gmail Multiple Login

Recently I and my colleagues are sending invitation letter to businesses for our Google Online Marketing Challenge and we are sharing a Gmail account that created just for this challenge. As usual, I will check the Sent Mail after a mail is sent, when I was looking at the Sent Mail, suddenly a new sent item appeared, and a few seconds later, another mail was sent and appeared on top of all sent mail. Mailbox hijacked? Of course no, one of my colleagues was logged in and sending mail at the same time. Just realized that Gmail(& others popular webmail as well) allow simultaneous login to one account from different computer or browser(session). Now I wonder is there a way to check or track simultaneous login...