I do have some slight recollection of MAC address on IoT being all the same, but might have been dreaming...
Two with the same (hardware) MAC address is unlikely in such a small order, but certainly not beyond credibility. Having 5 with the same MAC address is ridiculous. I'd look to the Win10IoT software for the reason for this.
Or they meant that both the WiFi and Ethernet on each RPi also had the same MAC address, that would definitely point to being a Win10 IoT software issue.
You mean Windows isn't the only OS?