> when people said something and I had learned before, instead of diminishing their statement by saying I knew that
I know someone like this. Their reply is always, "I'm not stupid". Clearly they were offended when I told them something they already knew, even though my intentions were to educate rather than offend. But their reply actually always created two offended people because to me, it's obvious I had no ill intentions and I'm left resenting their reply.
So now I just don't tell them anything unless they ask and I'm highly specific in my answer to their question. That ensures I'm only providing information they have indicated they don't know. If they want to know more, they'll ask more.
For my part, there was almost certainly an element of my being a know-it-all which is what got this person so riled up in the first place. Regardless; this simple change has helped me a lot and has improved my relationship with other people in addition to this one person.