I've been tracking this second transfer and I've discovered that was done with another addres. The first payment that I've sent you today create a new "change account" with the reamining btcs, and that was then used for the second payment. I didn't know that bitcoin-qt could do that. You can see it in the initial transaction
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The problem is that I have no control of these "change account".
bitcoin-qt keeps the private keys for any change accounts it makes, even if the corresponding public key is not shown in your address book. You should therefore have full control over this change address. If someone sends coins to it they will show up in your bitcoin-qt wallet, you can spend them, and you can use bitcoin-qt to sign a message using the key (just put in the associated public address).