;;;How to find out that a user is running more than one instance of a browser on the same machine?
Not aware of a way, cant think of why you would want to.
I use Use Session, or transient cookies and tend to steer clear of Permanent, persistent, or stored cookies. The only reason I would use the later is to offer the 'check this box to remember your username on next visit'
Get them to log in using information validated against a UN and PW stored in a database, then you have complete control.
;;;Is getting an IP address of the machine will solve the problem?
No multiple browsers on the same machine use the same IP address
Wind is your friend