I often write advice posts based on my own mistakes. I had a great one going, but it got too large and my memory promptly dumped the whole thing. Guess I've too many flaws serving me as inspiration.
But one thing that is important to do, and that I'm still learning, is when someone tells you something you already know, don't say "I already knew that" but instead say "Thank you" or "Thank you for sharing that." And don't say "Thank you for sharing that, but I already knew it." Too many times we should just stop with the "Thank you" without adding anything more.
That is something I plan to work on more this coming year, saying "Thank you" without more and without qualifying.