Sefer Chofetz Chaim hilchos Rechilus chapter 4

 One can transgress the prohibition of rechilus even if you don’t reveal anything new, for example if Reuvain played a trick on Shimon and Levi told 2 people what he saw. Then person A told Shimon Reuvain pulled a trick on him. This is for sure rechilus by person A but if person B then goes and tells Shimon also, that is also rechilus because Shimon might not have thought much about it after he was told once but he will definitely think more into once he was told about it a second time since it will stir hatred in Shimon’s heart for Reuvain. Another example is if Reuvain is found guilty in court and Shimon asks him what your verdict was, Reuvain tells him, and Shimon responds that doesn’t sound right. Even if he is just trying to give his own honest opinion it’s still forbidden, for one thing he only heard one side of the story, but even if he heard both sides of the story and knew all the information there is no point in telling the guilty party that the judges were wrong since the verdict is done and it’s too late. Even if he thinks he can fix the situation then still there is no reason to speak to the guilty party just go over to the judges and try to really fix the situation, maybe they will change their minds after they hear what he has to say but speaking to the guilty party only stokes the coals of hatred inside him. Back to the previous case, if person B adds more information to what Shimon already knows that is certainly forbidden also if Shimon wasn’t sure if what he heard was true and person B clarifies that also makes things worse. The way to fix the sin of rechilus and to repent is to apologize to the one spoken about and to admit your wrongdoing to Hashem, decide to try to never do it again and regret what you had said. Classic steps of repentance between man and man and man and Hashem. However, it should be noted that Rav Yisrael Salanter poskined that if the person spoken about does not know then one should not go over to him and tell him what you did and apologize because it will most likely make him feel bad even if you are trying to apologize. Making a person feel bad is worse. The Chofetz Chaim argues and says you should apologize anyways, however the Chofetz Chaim does give another word of advice which might be helpful for Rav Yisrael Salanter’s view which is if you did speak rechilus to someone then you should strategize and put in much effort to try to reverse the hatred you instilled in the listener’s heart, in that way you fix the sin as if it never happened.