Finally got a new party up to level 4 in Public build 0.4.21. I didn't check every feat for every character, but a couple of things jumped out at me:
The Might of the Iron Legion feat that grants Heavy armor and some weapon proficiencies correctly had a warning for "you already have some of these and so won't get the full benefit" when I previewed it with my Fighter. However, my Paladin did not get the warning, but should have as they also have all of the proficiencies already. Main difference is that my Fighter is an Elf (has some racial proficiencies that overlap) while my Paladin is a Human (only has the class proficiencies which are categories and not specific weapons so may not appear to match).
The other one that jumped out was everyone's favorite Sylvan Archer. My Elven Fighter already has a 20 DEX (yes, I might have re-rolled a few dozen times until I got an 18). However, once again, there was no warning marker, even though the feat states that the +1 DEX from the Feat has a max of 20. Since I was already at 20, that should not apply and therefore I expected to see the warning to let me know that I wouldn't get the full benefit. I did not select it to see if my DEX would actually cap at 20 or if it would go to 21 because I took a different feat.
I don't have a Dwarf with a 20 CON, but it would be interesting to see if all of the CON+1 (max 20) feats showed up with warnings for such a character or not. I suspect not, but haven't tried it. Has anyone else tried it and can comment?