So since 1.6.4 accessories can now be stacked without limitations. This is espcially strong when it comes to books that give resource bonuses. I am not really sure I like this change, since it will result in a) more snowballing (easy way to buff your pops in specialized cities to the max), b) situational books like the new food book or fortification becoming less attractive in comparison. It will also make the AI weaker in comparison since they dont stack at the moment I believe. On the other hand it gives additional strategical choice when it comes to city specialization, so its has benefits aswell.

I would say if you want stacking to be a thing, only allow stacking of accesories of the same TYPE but not the same level. This would have the following advantages: Stacking gets weaker, because - well - you cant stack 3 level 3 accessories anymore. Additionally a player that wants to focus that hard on one resource actually has to make an investment and get 3 corresponding armor technologies. Locking powerful effects behind investments is a good way to go about balance imho.