I was disconcerted to find that everyone else already knew it.

我发现别人都已知道此事，感到甚是尴尬。

《牛津词典》

I already knew it on my first day of primary school.

我在上小学的第一天就知道了。

youdao

After I'd read a couple of articles, I already knew everything there was to know about it (and more) : its advantages, its flaws and its potential.

在我读了一些文章之后，我已经知道了它的基本情况(或者更多)：它的优点，它的瑕疵，和它的潜力。

youdao

This being said, and I assume you already knew it, not every social platform are the same.

FORBES: An overview of the social media ecosystem

One of the things we learn in this article, I think we probably knew it already, there are many levels of clearance, not just secret and you wrote in the piece that if you studied all the secret programs, the people who are cleared for secret, it was just too vast to even do it.

NPR: Intelligence Failures In 'Top Secret America'

Well you all can decide if you like that phrase or not or maybe it was already in use and I never knew it.

FORBES: Taking It To The Streets - A Brilliant Example Of Street Savvy Marketing