I did a small experiment, I copied the wallet.dat and put it on another offline computer. I was able to send the same bitcoins twice! How can this be fixed?
It can be fixed by you coming back here and providing an update. Specifically, you could write an essay titled "Why I am not rich yet?".
To help you get started, here is a hint: you were
not able to send the same bitcoins twice, at least not any more than you would be able to send them twice by yelling "HERE, I SENT 'EM!!" from your balcony twice. Bitcoin transactions are verified, confirmed, timestamped, and entered into the ledger collectively by a p2p network.