I have just sent bitcoins from one account of mine to another, and after I sent it I checked on blockchain.info to get updated about confirmation. There, I noticed something peculiar, it said that I made two transactions at the same time, when I made only one. The first transaction was to the correct address and the correct amount. But the second transaction was not one that I made. The second thing I noticed is the SUM of both transactions, together they form the exact amount I last received to this account a few days ago. This looks to me like a hack. At this moment these transactions have net yet been confirmed, can I do anything about it? Can a transaction be created fraudulently and accepted by the miners? How can a hack like this be accomplished (if it is a hack).
You are not being hacked. These are not two separate transactions; it is one transaction with two outputs. The second output was automatically created by your wallet client and sends back to a new address which also belongs to you. This is a normal and necessary feature of how Bitcoin works, called "change". See How does change work in a bitcoin transaction?
If this transaction were malicious, it would mean that someone had access to your private key; if so, they would also have stolen your earlier transaction. Moreover, they would have stolen the coins as soon as they obtained the key; they wouldn't have waited for you to make your own transaction.