Some of my client duplicate their MAC address of their LAN card and use 2 or more PC to connect internet. As I use DHCP how can i prevent to connect from multiple PC, and specify the pc that i allow will only be able to connect to internet and others are not.

As you are using DHCP, You can always make the LEASE as a "static Lease", where you can bind IP address with Mac -Address.

i use static lease for one ip and one mac. but client duplicate their mac to another nic then use 2 pc at a time. now dhcp lease the same ip to 2 pc. how can i solve it?