The following is based on an article written by Rav Avrohom Genechovski [one of the greatest Talmidei Chachomim on the planet. A genius of magnificent proportions] in the Habe'er journal [5764].
Reuven told Shimon a secret and asked him to keep it a secret. Shimon told Levi the secret and Levi reported back to Reuven that Shimon related the secret. Is Reuven permitted to rebuke Shimon? If he does, Shimon will understand that Levi told Reuven what Shimon had done. Telling "A" what "B" did to him or said about him is a biblical prohibition called "rechilus". So here maybe it is rechilus about Levi if Reuven rebukes Shimon.
The goal of Reuven is not to harm Levi but to rebuke Shimon, but nevertheless he is [indirectly] saying rechilus about Levi. Even though he doesn't intend it as rechilus and one could argue that it is a "davar she'aino miskavein" an unintended prohibition which is permitted on Shabbos - maybe davar she'aino miskavein isn't permitted in the realm of interpersonal relationships.
Maybe one could argue that is permitted because there is a benefit [toeles] and rechilus is permitted li'toeles. But here the toeles is for the teller and that is not permitted, just as if a boss says to his employee "Either tell me lashon hara or I will fire you" he is not allowed to tell because it is for his own personal benefit.
Maybe Reuven can tell Shimon that a person who overheard Shimon telling the secret to Levi told him [Reuven], but that would only work if Shimon would buy into the story. If Levi says that he doesn't remember if anyone overheard Shimon speaking, there is also the possibility that Shimon told other people so he wouldn't automatically know it was Levi. This would be a classic case of sfek sfeika - double doubt, which is normally permitted. But we NEVER find that in areas of interpersonal relationships sfek sfeikas are permitted.
Bottom line - the halacha is not clear.
So keep secrets - secret.
:)