If a person starts telling you, whether in private or public, something that you already knew very well, you should pretend as if you do not know it. Do not rush to reveal your knowledge or to interfere with the speech.
Instead, show your attention and concentration. The honorable tab'i Imam Ata ibn Abi Rabah said: "A young man would tell me something that I may have heard before he was born. Nevertheless, I would listen to him as if I had never heard it before."
Khalid ibn Safwan al-Tamimi, who frequented the courts of two Khalifahs: Umar ibn Abdul Aziz and Hisham ibn Abdul Malik, said: "If a person tells you something you have heard before, or news that you already learned, do not interrupt him to exhibit your knowledge to those present. This is rude and ill mannered." The honorable Imam Abdullah ibn Wahab al-Qurashi al-Masri, a companion of Imam Malik, Al-Laith ibn Sad and Al-Thawri, said: "Sometimes a person would tell me a story that I have heard before his parents had wed. Yet, I listened as if I have never heard it before." Ibrahim ibn al-Junaid said: "A wise man said to his son: 'Learn the art of listening as you learn the art of speaking.'" Listening well means maintaining eye contact, allowing the speaker to finish the spech, and restraining your urge to interrupt his speech.
Al-Hafiz al-Khatib al-Baghdadi said in a poem: Never interrupt a talk, though you know it inside out! [Source: Islaam.com]