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.


Depends who it is. I have neurodivergent friends who can’t drop a topic before they’ve spoken their final thought on it. I’ll listen to the end unless they’re venting thoughts that should remain private in public, in which case I’ll shush them.
Other people - the first time it happens, I’ll be polite. If it keeps happening, no.