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.

Totally depends on the scenario, I use all 3. If we’re talking about music, hobbies, personal interest type stuff, it’s usually C, without the “tell them you already know”. I try to show that I know instead of telling. Like if someone’s telling me about a band I already know about, I’ll let them finish and then say something that requires deep understanding of the source. If the discussion is academic, B, but like in a respectful way? Not ‘cutting off’ the other person, more so ‘jumping in’. And that’s so we can both establish what we both know and move on to the more interesting stuff. A is used for authority figures that I have no respect for. I have nothing to say to marching orders.

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.

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!

How long are they going?

I get excited and finish their sentences, then try to take the convo past the basic stuff, but try to share in their interest.

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.

Depends. If they’re being a condescending twat, I’ll brusquely tell them I already know more than they do. Even if I don’t.

That really depends on whether I’m trying to get on their good side at that time.

I normally pretend to not know anything about it, I either don’t correct them. Just listen and use what’s useful to my benefit and ignore what’s incorrect. No one wants to talk with someone who knows everything about a topic.

I listen very carefully for any mistakes they make so I can correct them.

D) Excitedly finish their sentence to create a moment of shared understanding

“So you have xyz and then–”

“Oh yeah! B! It’s so cool, isn’t it? And then C and D!”

“Yeah!”

Fist bump

Or at least this is what I would want to happen if I ever actually talked to people. >.>

I rarely find myself in this scenario, but depending on the situation I’ll either nod along and wait to add any points I have or will cut them off with a question.

Depends on the context of the conversation.

If it’s a casual conversation and they are enthusiastic I’ll let them talk.

If someone is trying to tell me how something is done while they quote the manual I wrote, I’ll cut them off.

If they really think you don’t know then interrupt and say that you know. If they just want to be listened to, which happens a lot, then listen if that’s ok for the situation.