Blockchain double spent my coin, need help

[codesphere]

Hi!

I was depositing into localbitcoins, 10 minutes after my initial deposit, I heard another *peep* from my blockchain and saw that another transaction was made without me doing anything.
Few minutes after that, the second transaction got it's first confirmation. Bitcoin has been taken from my account for both transactions and as i've heard, I will not be able to cancel this.

Even more weird is this that blockchain made the transaction while I even didn't have enough coin, so it made my address negative (-0.4 btc)

This is relatively big amount and I need some advice how can I get my money back?

Thanks

[ranochigo]

I have a suspicion that this is caused by the recent wave of malleability [1] attack on the blockchain. Even if the transaction is broadcasted, nodes can still change the TXID such that it would appear different. No worries, the security of the transaction isn't changed, the amount is still going towards the correct person. However, your client might have been confused by it and recorded it twice hence the negative balance. Try to contact blockchain.info support for help.

[1]https://bitcointalk.org/index.php?topic=1198032.0