My internet provider has done static binding to thee MAC address of my adapter. Now I need to use the same internet connection on another machine also.
So, I figure that one way of doing that is to change the mac address of my second machine to that of my first machine (making both of them same).
Is there any harm in doing that? Or any reasons this is not advised?
Well, your second computer will also get all of the traffic intended for your first computer. But that shouldn’t be a problem since the NIC should filter out the traffic headed for your other computer (based on the IP in the packets). A better idea would be to get a router, set its mac address to that of your first computer, and then connect to it from both of your other computers. The other advantage would be that you should be able to connect with other computers, too.
If both of those computers are on the same network then yeah having duplicate MAC addresses is a bad idea. The whole idea of MAC addresses is that they are unique for network identification purposes (long before your IP stack comes into play).
Check out https://serverfault.com/questions/88830/can-duplicate-mac-addresses-on-same-lan-cause-trouble for some insight via the answers to that similar question.
To rephrase the other answers to more directly address your question – no, there is no harm in cloning the MAC address as long as you make sure that both machines aren’t plugged into the same Ethernet at the same time.
If you’ve got Machine A plugged into your cable modem or DSL router, and you’re going to unplug Machine A and plug in Machine B instead, this should work fine.
But, as people have said in this question and in response to your other question, the right way to address your situation is to buy a simple router that does NAT, let your ISP bind to the MAC of the router, and then you can do whatever you like on your LAN and your ISP won’t know or care.