As it is, you can only equip either shirt + duster/ jacket or shirt + armor.
My suggestion is adding a "vest" or "body armor" so you can have shirt +sleeveless armor/ vest + duster/ jacket, by sleevless armor I mean items like: Riot Vest or Reinforced Vest ,full body armor like Hexweave/mesoprene plate or Laminate armor would still take up both "back" slot AND the new "vest/ armor" slot.
The revised "back" slot (could also be renamed to "coat/ jacket") could then only be used for non-closed coats/ clothing like coats (Isnt there a word for this?) , such as Greaser Jacket or Duster , to prevent stacking armor on top of each other. Of course the armor stacking problem can also be solved by making this whole system wardrobe only.