(Before you scream "21 million": that bitcoin limit is "guaranteed" only by fuzzy arguments about a complicated economic game, not "by math", and could be changed if the right players agreed to it. Moreover, any kid can duplicate the amount of bitcoins in existence by creating a hard fork of the blockchain and starting to mine it on his laptotp. Anyone who has bitcoins will gain an equal amount of those "series B" bitcoins, accessible through the same private keys, and could trade them independently of his old bitcoins by duplicating his wallet and downloading the kids's client software. Whether those "series B" bitcoins will get a significant market value is a market(ing) question, not a technical one. And, of course, there are the altcoins.)
You're describing a double spend attack, right? Ether that, or something completely idiotic.
Imma go ahead and call bullshit. There's no way you can pre-mine a blockchain, you're racing against the entire network to solve the hash. The hash of the previous block is included into the hashing process of the next. You'd have to start out solving the latest block on the chain, and the consecutively solve... how many blocks do propose, exactly? Not only that, but pre-mine
ahead of the network. The most consecutive blocks mined are six, by the largest pool in the entire network. This isnt a "any kid can do it at home with his laptop" scenario.
Then, just copy over the wallet to some other computer and spend the coins twice... What?
You obviously have not idea what you're talking about.
I'll bet you all my coins (0.107) that
you (you consider yourself superior to "any kid with a laptop", right) definitely
CANNOT duplicate any bitcoin through the method you described.