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.
Assuming they are not doing some kind of smug petty explaining they know I already know and there is plenty of time available, I will listen to see if there is something I don’t know about or whether they have any gaps that I could fill in. I mean, talking about things one already knows about is really common.
If they are smugsplaining something I will just cut them off and move the conversation along. If there isn’t time I will say that I am familiar with the topic and ask if they could focus on the relevant parts.