Apply a bunch of different modification on the same helmet:
Start adding them to a chest/vault, watch them stack based on the type of modification kit applied, even though they have all different modifications applied:
Take one of the stacked ones out:
Receive a new not modified item. If a durability kit was applied, the new not modified item keeps the increased durability:
I cant figure out yet what the stacking pattern is but for example: Gigling Joints stacks with Bulked Plating, Simple Armor Flexibility Kit with Advanced Flexbility Kit, Master Armor Plating with Armor Plating but not with Thick Plating.
Repeat with Crom Sword with various fittings and kits:
Observe a similar effect:
New plain Sword of Crom with 2850 durability vs 2500 durability on a new sword: