Shemiras Halashon Halachos

Hilchos Rechilus, chapter 4, section 1

The prohibition against rechilus can apply even if one reveals nothing new, such as if the person spoken to already knew that so and so had spoken evil of him but by now speaking rechilus to him one causes him to think about the event and realize that so and so did him harm.

Hilchos Rechilus, chapter 4, section 2 3

If Reuven denigrated Shimon in front of two people, and one of them informed Shimon, the second should still not speak of the episode to Shimon. This applies not even when Shimon is unsure whether the story is true, for example, if he asked the second person if Reuven really denigrated him, but even generally. Because repeating the story increases Shimon's hate for Shimon. To repent the sin of rechilus, one must appease the person spoken of, regret what one did, confess the sin, and accept upon oneself wholeheartedly not to do it again. If one clearly knows that no harm was caused by the rechilus, one does not need to appease the person spoken of.