The phrase "known knowns" means things that we already know and are aware of. It refers to information or knowledge that is familiar to us and that we have already recognized or acknowledged.
Sentences with «known knowns»
You may be smart... but also know you know you can be a lot smarter. (businessinsider.com)
After that, I have enough experience to know no matter what you do, we have always people who question what you do — that's normal. (justarsenal.com)