OK so I just duplicate something by holding option and drag it to somewhere, no problem so far.
But when I want to undo this, I need to cmd + z "2 times" in order to 1. move back the duplicated one to the original position (A "transform" record in History) AND 2. Delete the duplicated one (A "duplicate" record in History).
This is NOT A BUG literally, but it's a little bit annoying and counterintuitive. Because, to me personally, I tend to subconsciously think "holding option and drag" as "one step", but it takes "two steps" to undo. Hmmm... so weird.
I'm not sure if anyone has the same feeling :( or if I missed something please let me know :) :) :)
(I think Adobe illustrator does better in this case though)