MAC ADDRESSES - CAN THERE BE DUPLICATES

We have a network of over 100 PC's with all fixed IP Addresses.  We provide a limited number of DHCP addresse for our laptop users. 

The problem is that at various times during the day , night and week-ends, the mac address associated with the LAN adapters on two particular PC's, a Dell Optiplex 170L and a Dell Optiplex 780 , are going out and also grabbing DHCP addresses.

Anyone know what this could be.  Is it the PC's other hidden network adapters that are grabbing DHCP addresses, or could there really be duplicate MAC addresses on other devices?

I regularly monitor the DHCP logs.  The MAC addresses appear, renewing a DHCP address but with no identifier.  "Renew,192.13.10.120,,001E4FF57808".  This MAC address is also the MAC address for the PC's Network LAN Adapter.