If a judge issues a ruling, having tried his best to decide correctly, and his ruling is right, he will have a double reward, but if he issues a ruling, having tried his best to decide correctly, and his ruling is wrong, he will have a single reward

This Hadīth points out that if a judge tries his best concerning a certain case until he reaches what he thinks is the right decision, and then he gives his ruling accordingly, he will have a double reward if his ruling is sound and correct, and conforms to what is willed by Allah, the Almighty, from His rulings. One reward is for his effort and keenness and the other for reaching the right decision.
However, if he makes his best effort and does not reach a correct ruling, he will have a single reward for his diligence. Indeed, the effort exerted in pursuit of the truth is in itself an act of worship. A judge who earnestly endeavors to reach the correct decision does not incur a sin if he makes a wrong one, provided that he is knowledgeable and qualified for this endeavor.