At first glance, in a scenario where Reuvain tells two people that Shimon is stupid, for example, and person A tells Shimon that Reuvain called him stupid which is rechilus and the question is if person B now tells Shimon what Reuvain said has he transgressed rechilus, the Chofetz Chaim says is based on an argument between Tosfos, the Rambam and Rashba vs. The Rosh and Nimukey Yosef whether a person who digs a pit already 10 tefachim to 20 tefachim deep is liable if an animal falls in and dies. Tosfos, Rambam, and Rashba hold the person who dug the next 10 tefachim to 20 is also liable because he also created a damage that could potentially kill the animal if the first ten tefachim were not dug, therefore so to by rechilus we might be able to say that since his statement could have had the same damage as if the first one was not said the he is also liable for rechilus. The Rosh and Nimukey Yosef hold that since in reality the second person didn’t add anything then he is exempt for damages, and this could be true by rechilus as well. The Chofetz Chaim did say that even those who would say the damage would be liable that is only true if nothing has happened yet and you can potentially say either one of them could cause the damage so both are liable but here the damage was already caused the rechilus was already said so it’s like adding wood to a fire which already burnt down a house to ashes which should be exempt because nothing was done.
However all this is in a technical world if you can evaluate that no damage is being added but the reality is that ideally the second person cannot repeat what he heard to Shimon because usually what happens is that Shimon might have brushed off what the first person said but when he hears from someone else that Reuvain called him stupid then that not only gives more validation to what was said that it was in fact said but it stokes the coals more and entices Shimon to now go over to Reuvain and pick a fight over what he seems to have said. In fact, the second person’s repetition of what the first person said is in fact a worse degree of rechilus because of its reinforcement of what was previously said and gives more credibility to the rechilus which means it is certainly forbidden according to everyone.