What happens if you copied private key and try to use it again? I am very new to bitcoins. I have one question. Suppose some one copied their private key and saved it in their harddisk. Next they will do the transaction. Now since you already copied the private keys can you use it again?

How bitcoin prevent this?

I am asking this because. I wish to create something smaller for my brothers  school project. So I am out of ideas on how to prevent this.
Every private key owner can spend any amount of coins on appropriate address, but every coin on any address can be spent only once. After spending the coin it is suggested to go into another address which has another private key.

upd: Technically they are stored on the blockchain file (>200GB now), which of course is able to copy and wanted to be copied, but your private key is the only ability to let your coins be moved in the chain - if (and only if) you have the proper key. So, the proper key is hard to get and easy to lose and you might not like the idea to copy it except of backup operation.