The Chofetz Chaim went into great detail to explain why someone else cannot repeat the rechilus that the first person already told like in a case where Revain calls Shimon stupid in front of two people and Person A unlawfully tells Shimon that Reuvain called him stupid. Person B cannot also tell Shimon that he was called stupid by Reuvain even if he did not add anything more to what happened. Why not, the damage was already done, what is he adding? Even more so we find that if something was said in front of 3 then it can be repeated because, it will get back to the subject spoken about. The fact that he said it in a fashion that it will automatically spread must mean he did not have any qualms saying it. The statement might sound shady, but it is not out right outlandish, therefore if said in a way that it sounds like it was meant to be spread it must not be that bad and is permissible to repeat. If so, then once something was already repeated then why can’t it be said again, if there is no harm done since it is already out? Granted our case is an outright injustice of a statement and only 2 people heard it but once it was repeated then why can’t the other say it again the same way, nothing added, what harm is there?
There is an episode in Tana”ch where Nasan the Prophet tells King David that his son, Adoniya, has started a rebellion and declared himself king. And Nosson told King David this after Batsheva, King Davids why and mother to his heir, Shlomo. So one might say we see from this episode that you can repeat something already told by someone else. However, Nosson the Prophet went a step further and added information like how Eviyatar was in cahoots with Adoniya which Batsheva never mentioned, therefore it must be he was only allowed to say this lashon hara because of what the Yerushalmi in Peah chapter 1remarked as a halacha that one may speak lashon hara about two people who are fight against each other. The reason being is that by speaking out and publicizing the matter it is possible that you can apply enough pressure to stop the fight for the sake of peace.
The Chofetz Chaim then interestingly applied this halacha to a case of damaging in Bava Kamma. There is a whole question if one adds wood to a preexisting fire which burning down a house is he liable for causing damage. If it contributes and strengthens the fire, then certain he is responsible but the gemara concludes that if he just through a twig into a blazing fiery inferno he is exempt because he did not add anything to make the damage worse. The Chofetz Chaim wants to say then that that will be a proof one can repeat something which does not add in any way to whom the bad mouthing he was told about before hand had said. It’s the same words and connotation.