It is forbidden to speak in a derogatory manner even if the information being conveyed was already known to the listener. This is because the act of speaking negatively is inherently wrong.
Conversely, if stands to reason that a non-derogatory but potentially harmful statement can be made in the presence of anyone who is already aware of the information, unless the speaker emphasizes some point that the listener may not have realized previously. This is because the statement will not be cause for any harm which otherwise would not have come about.
Rambam takes this a step further, stating that if a potentially harmful statement was made in the presence of three people, the three are permitted to relate the information in conversation with others. This is because each of the three can assume that the other two will probably make the information public anyway. However, this would be not be permissible if either
(1) one of the three is known to be discreet or especially careful regarding the laws of forbidden speech, or
(2) the person’s intent in relating the information is to publicize the matter yet further.
According to the Chofetz Chaim, Rambam permits any of the three to repeat the information even if it is derogatory.
Sefer Chofetz Chaim, 24 Tishrei, page 96