Hilchos Rechilus, chapter 4, section 2 3

If Reuven put down Shimon in front of two people, and one of them told Shimon, the second person should still not talk to Shimon about it.

This is true not only when Shimon is unsure whether the story is true (for example, he asks the second person whether Reuven really put him down), but in every case. Repeating the story makes Shimon hate Reuven even more.

To do teshuva (repent) for the sin of rechilus, you must make peace with the person who was spoken about, regret what you did, confess the sin, and firmly decide never to do it again.

If you clearly know that the rechilus caused no harm, you do not need to make peace with the person who was spoken about.