I’m thinking of 2 scenario that someone can ‘abuse’ the network. Wondering if Subspace is ready for tackle down?
#1: if someone copy the node’s db to new PC, and he will plot from 0 in that PC. Is it allowed? Does it confuse the consensus? Can it cause any possible harm to the network’s consensus?
#2: if someone copy the node’s db and the same plot to new PC, so at same time, he’ll have doubled proof from 2 different physical PC’s. Can we detect that action immediately?
Consensus can’t know anything about this and doesn’t care. The only thing you save here is sync time, but you better ensure you get the database from a trusted source (like a database you have created yourself on a different machine). If database is manipulated in malicious way you may end up on a fork as an example of something bad that could happen. If this is a one-off issue there is no concern for the network, but if majority do it then manipulating the source of these databases can have bad consequences.
Node’s database doesn’t matter. Running two copies of the same plot on two machines will result in equivocation as soon as two solutions for the same slot are announced to the network and the plot will be burned on consensus level. Farmers tried it on previous networks and learned the hard way 
Thanks Nazar. So only #2 is not allowed. So the consequence is the whole plot will be burned, i.e. he will never win any reward with that plot even he turns off the other node and keep only one node/plot online?
They should get the very first reward, after which they’ll get nothing else because blockchain will remember their plot as burned permanently. Doesn’t matter if they are online or not anymore.