What happens if you copied private key and try to use it again?
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.