There is a section "Online Customers" under Customers in Admin.
Here magento shows name, email, ip address, session start time, last activity and last url accessed.
you can call 'log/visitor_online' model and get list of all currently online customers/ visitors.
Now whenever a customer logs in, you can make a custom observer and call its function to match email of that customer with all this data you got from model.
and logout cutomer from previous device if you find a record same.
Hope you understand my point.