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.
I would only interrupt to ask clarifying questions to see how far their knowledge goes. There’s many ways to come to know about a thing. Then I might learn something new!
Aw you’re such a saint