Many do, some don't. Moreover, the MAC address is volatile in that it
can be changed at will (via, eg, ifconfig).
I assume that the reason that Sun (for example) defaults to all MAC
addresses on a system being the same is that it doesn't make sense,
ordinarily, to plug two Ethernet interfaces into the same network
segment. If, for some reason, you really want to do that, there's
ifconfig ready to reassign the MAC address.
If I plug both into the same network segments by accident (because I
can't tell which is which, say), then my configuration is nearly as
broken with different MAC addresses as with identical ones; the fix
is to replug correctly, not to change MAC addresses.
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