Is he considered a sinner, having withdrawn a sum of money on behalf of his sick maternal aunt after the state had paid her twice, upon her request, knowing that the aunt has passed away?

If the recipient withdraws money that was paid to him twice by mistake, and he is aware of the situation, then he is sinful for cooperating in sin and aggression and for consuming wealth unlawfully. He must inform the deceased's heirs so that they can inform the granting authority of what happened and return the money, or absolve their deceased relative of it.