This appears to be the result of using a faulty wallet program. What operating system are you accessing blockchain.info from? Are you using a web browser or an installed app?
When generating the necessary digital signatures to broadcast the transaction, the wallet re-used a value that is supposed to be unique for each signature. This allows a hacker to calculate the private key from the two signatures. This is one of the reasons that it is a good idea to always use a new address for every transaction. If the change had been sent to a new address (like the way Bitcoin Core handles change), then this attack wouldn't be possible.
Your 1PNa9dZ3P3fVhx1uMCqJ4sEYmyhxnQNy3M address is listed here:
Hello,
there has been a lot of reused R values in the signatures on the blockchain, recently. This exposed many private keys. After googleing the addresses, I think it is related to Counterparty (XCP). Here is a list of the exposed addresses in alphabetic order. Most keys were exposed very recently, i.e., in the last week.
If you own one of the following addresses, you should transfer the money to a fresh address (before someone else does it for you). Also figure out, which client has the bug that revealed the private key by reusing R values. Then notify the author of that tool.
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1PNa9dZ3P3fVhx1uMCqJ4sEYmyhxnQNy3M
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