The Chofetz Chaim offers another reason why we should not believe loshon hora which is said in our presence.
When a person speaks loshon hora, he transgresses the negative commandment of “You should not go as a peddler of gossip” (Vayikra19:16), thereby putting himself in the category of a rasha (evil person). As a rasha his words certainly have no credibility, and we may suspect him of lying, exaggerating, and distorting the truth. Furthermore, this wicked individual is telling us negative information about someone who is assumed to be an upstanding, observant Jew! Certainly we should not accept his wicked words as fact.
If we hear the same negative information from two or more people, we may be more inclined to believe it. This is incorrect, says the Chofetz Chaim, because when wicked people speak wicked words, numbers are meaningless. Even if a dozen people are offering the same derogatory information, it should not be accepted.
The Chofetz Chaim adds that this halachah (law) applies even when the two speakers are not deemed reshaim (wicked people). For example, suppose two people approach Levi in the street and inform him that Yehuda is planning to ruin his business. If they are telling the truth, then they are actually doing a mitzvah by warning Levi. Nevertheless, Levi can only protect himself on the chance that the report is true; he cannot accept it as fact.
This is because the testimony of two people has validity only in beis din (rabbinical court). When two people report negative information about someone outside of beis din, they are not restrained by the possibility of being branded as false witnesses, for there can be no such designation outside of beis din. Therefore, their report cannot be accepted as fact.
If a rumor circulates in a city that a Jew committed a crime, one is not allowed to believe it. This applies also to reports in newspapers or other media sources. In this case, too, if the information is relevant for constructive purposes, one should proceed with appropriate caution.
However, there are instances in which one may believe negative reports. When an abundance of reports regarding a certain person circulate over a period of time, telling of various sinful acts which he committed, to the point where he is no longer viewed as an observant Jew, then it would be permissible to believe the reports. As the Chofetz Chaim puts it, we are not required to think that the community has made a mistake again and again regarding the same individual.