Or
B) Get impatient, cut them off and tell them everything you know about the topic?
Or
C) Patiently let them finish, THEN tell them you already know, and everything you know about the topic?
I personally have a tendency to be an impatient douche sometimes so I admit I don’t always handle these situations with the greatest patience & social grace.
“oh x aspect you say? isn’t that kinda like c, d, y, z, aspect” or similar to politely as possible state what I already know so we can either skip past those parts or disagree/clarify as the conversation requires.