Hyper-V Program Manager
Just wanted to pass this one a long, is embarrassingly enough I actually learned about this on one of our public newsgroups. By default Virtual Server 2005R2 will detect if you have virtual machines on the same box configured with duplicate MAC addresses and stop you from starting them up at the same time. Now for 90% of cases this is the correct thing to do. However, if you really, really, really know what you are doing and need to boot virtual machines with the same MAC address you can do the following:
You can now start virtual machines with duplicate MAC addresses, but once again please be careful when doing this.
So... this just begs the question, why might one want to run two (or more) VM's with the same MAC address?
Well, you can configure more than one virtual network...
I can imagine the situation where you have some VM in a production, which is connected to "External Network". Now, you want to accomplish some tests. You make a copy of the VM in question and connect it to a closed "internal" virtual network... You will want to keep the MAC address.