It's not actually someone using your account. They happen to have a similar MAC address to you and due to a glitch in the system the machines are appearing as phantoms on each others' accounts but only can access their own account..
Please contact Customer Service regading this issue.
The link is under Useful Links at the top of this page.
We've been told on our conference call today that the problem is being worked on but those phantom users cannot see your account details.