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.
In my opinion: kind of option B)
The situation you describe is one of those moments where it’s actually appropriate to finish their sentence - so listen to the other person and find an opportunity to do so, or comment (usually somewhat enthusiastically) “Yes, I’m familiar with that!” Possibly adding a short anecdote of how you became familiar with the topic.
They’re still going to want to discuss it regardless, but this way it’s a more productive experience because they now understand that you’re more up to their level of knowledge. So they can move on to higher ideas about the topic rather than get bogged down in exposition.
It is not rude, because you’re showing enthusiasm for a topic they’re sharing with you. People prefer this to spending the time telling someone something they already know.
I would argue that waiting for them to finish can be ruder because it might make it seem like you were focusing on something else while they spoke to a brick wall.