Ok got it. It was my mistake. I thought coins are stored in your computer with every bitcoin as its own address. Now I understand it perfectly. private key is only for wallet address. Not for bitcoin

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? 

Thanks in advance for clearing my doubt.