If you go to Site Admin > Plugins > Authentication > Manage Authentication, you can limit the number of concurrent logins for a user.
By default, there is no limit to the number of concurrent browser logins. (If a user is concerned about this, they can view their Browser sessions page.) This setting allows you to specify how many concurrent browser logins are allowed. Once the limit is reached, the oldest session will be terminated. Note that this does not work with Single sign on plugins.