If two systems have the similar MAC address what will the switch do

If two systems A&B have the similar MAC address and system C is trying to pig A. Assume both A & B had unique ip address and all the 3 machines are connected to a switch. Then will the switch do? How it functions and takes the decision?

We can change the MAC address if it comprises of EPROM. So rule out the issue that we cant change the MAC address. Try to figure out the exact answer. How does the switch takes decision when it encounters such a situation?

Check this link to change the MAC address for a linux machine(If u’r NIC supports).
http://www.linuxsecurity.com/docs/Hack-FAQ/data-networks/change-mac-address.shtml