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Not in ESX , in HyperV. ESX is usually pretty good.

I have that before. PVS has device that has the mas already.

yea had that before when creating desktops in the thousands and one of the xendesktop setup wizards fail before it registers the new devices in the last step with the machine catalog. you have to look at the either pvs or vsphere to figure out the last item that was created (can't remember which way round they are created now, probably PVS device first then vSphere VM), grab the mac address and do a search to ensure that the mac address is being used elsewhere. we script ed using citrix snapins and powercli the mass deletion of the vm's as we found it easier just to delete everything in the failed attempt range and start again. If you find one duplicate then it seems likely that there will be others, so the XenDesktop setup wizard would fail again and just make the issue worse.

We ended up tracking this down to another cluster actually made the same MAC randomly