CentOS - Jun 2015 - why network interface had same mac address in pxe mode

Hi all:
        when I reinstall os by cobbler on the machine which two network
interfaces config to bond mode=0, and system is centos 7.1 ,
       I saw this error:
       https://github.com/cobbler/cobbler/issues/1478
<https://github.com/cobbler/cobbler/issues/1478>
     
         finally,  I unplug one network interface and pxe work well !
         what I confuseed is  why  network interfaces  do not restore to
original  mac  in pxe mode   ?
         why the two network interfaces has same mac address even on pxe mode? 
         what can trigger this problem ?




thank you
??????
Firxiao