The strangest problem in the world, ever!
This problem has dragged my back to edgeek, not necessarily for an answer just to share my pain.
We have an NEC Flex server running a hyper-v cluster on 2008 R2 and the server with the fault is also 2008 R2.
I came in Monday morning and one of our physical hosts was unreachable so I rebooted it and everything seemed to be back up, then I noticed that all traffic being sent to the virtual servers that were on it is being duplicated.
For example, if I send a ping to the server running wireshark I see these results.
On my machine: 4 pings sent, 8 replies received.
On the VM: 8 pings received, 8 replies sent.
This is only happening with the virtual servers and not the hardware hosts.