Quote:
	
	
		| 
					Originally Posted by rkruz  I know all MACs are supposed to be unique.....but....what happens on a LAN if two adapters have identical addresses? | 
	
 MAC Addresses are controlled by the Institute of Electrical and Electronics Engineers, it is a 48 bit hexadecimal address the first 24 bits are assigned by the Institute of Electrical and Electronics Engineers as a vendor code, the other 24 bits are for the vendor to give to each Network Interface Card a unique address.  there cannot be two Network Interface Cards with the same MAC address.
of course there is MAC address spoofing which can clone a MAC address which can complicates things.
Two hosts with the same IP address on the same network would cause a conflict as Jon T has pointed out.