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MAC Addr's are most assuredly NOT unique. I know this because, many moons ago, I had two 3COM 3C509's on the same segment that were having all kinds of network problems.
Until I just happened to glace at the MAC Addr bar code at the top of the cards. Yep, identical. If you think about it, out of the 6 bytes of a MAC-48 address, the first 3 are used as a manufacturer code. The remaining 3 bytes are a sequence number in the production, giving you 16.5 million possible addresses. What are the chances a manufacturer is going to make more than 16.5 million Ethernet adapters?? You can have two adapters with the same MAC address on the same network, but NOT in the same segment. The MAC (or physical) address is what is used to communicate between two nodes on the same segment.
On top of that, yes, you can change the MAC address of certain adapters, thereby opening yourself up to spoofing and physical address collisions. Mkvii Gti Installing Rear License Plate Bracket.