my friend asked me "so, I send you a coin from my BTC account, and you get it and it is verified... but between the time I send it, and you verify it, what is stopping me from sending a copy of that coin elsewhere?". This assumes a user could hack the wallet and make a duplicate coin, which I have no idea how possible that would be (what DOES prevent me from hacking my wallet?)
Can someone share with me what the correct answer to that question is?
Thanks
What stops him is that the network is constantly communicating and minting new blocks to record transactions. One transaction will make it into a block, the other will be rejected as an invalid double spend by any node that possesses that block.