it really annoys me when I'm having a conversation and when someone is like saying things and showing me things that I already loving know about
an easy way to avoid this is to mention something related to the thing you want to tell them about or show them without acting as if they don't know about it
like in a conversation on how to make some recipe for chocolate cake or some stuff, a bad way of telling your friend about it if he already knows is to pretty much tell him how to do it and the end result, a good example would be to say something like "bro this chocolate cake recipe is awesome", but say your friend didn't know about it, if he's curious and wants to know, he'll more than likely ask about it
so it really works for both situations in whether or not your friend knows about it
just a little thought of mine ;)