Lesson 1 If someone made a derogatory remark about another person, you are not allowed to repeat this remark to the subject, even if you feel that you would have done so all the same in the presence of the person you are quoting. This applies all the more so when the originator of the remark is actually present. The listener will more readily accept the statement to be true and much hatred will be caused.
Lesson 2 If you are told that someone spoke lashon hora about you. - you are not allowed to go to the initial speaker and ask "why did you speak against me to that person?"