Hey.
I wanted to create kit with gold spade which had 0 durability. Everything is fine. Then I tried to get kit twice and items were stacked. Thats huge problem in essentials in general that items like armour,tools and potions are beeing stacked instead of beeing placed in other slots.
When I got 2 spades with 0 durability stacked I broke block for funz and 1 spade disappeared (logic) BUT the 2nd one had max durability.

^ this is based on permission.

Kits do not stack for me. Only when having the stack permission.