(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?
No, I am describing a hard fork.
It does not interfere with the original chain directly, simply creates another clone, premined and "pre-transacted" to hae the same addresses, private keys, UTXOS, and everything as the original chain. All it can do is steal value from the original bitcoins, if for some reason people prefer to spend their dollars on "series B" bitcoins rather than original ones.
What tosh. Who would spend their dollars on series B bitcoin, what scenario do you envisage where someone would be tricked into buying from a second chain.