What gonna happen if two geth clients( in two different machines) running with the same nodekey file (node id) within the Mainnet?

Question

If someone copies the nodekey to another machine, and runs the geth with that key. Test it, just don't quite understand the underlying mech.

Answer 1

If two nodes are using the same node key, the network will only recognize one of them and only one of them will be able to broadcast to/work with the network.

While this is happening, the other one may be running and synced, but it will be "invisible" to the network.