If manufacturers start duplicating MAC addresses in manufacturing runs then a lot of customers would end up with tons of problems. It sounds unlikely to me that they do, or customers would have raised hell by now. We would have had trouble at work, as we buy batches of computers all the time. We don't fiddle with the MAC address. We register them though, then we plug all the new boxes into the network. No dups so far.. If a practice of producing equipment with unchanging MAC addresses exists then it doesn't seem to be followed by vendors like HP or Dell, at least.