(You can find a lot of articles on this from the internet. I just put it here for my own record.)
It's because the cloned system has the original mac address in the Linux configuration files.
1. rm -f /etc/udev/rules.d/70-persistent-net.rules
2. vi /etc/sysconfig/networking/devices/ifcfg-eth0
Remove the MACADDR and UUID lines.
3. reboot