How to reproduce

/summon armor_stand
/replaceitem entity @e[type=armor_stand] slot.weapon.mainhand 0 diamond_chestplate

Expected result
Armor stand is holding chestplate in hand

Actual result
Armor stand is holding chestplate in hand and wearing one on its body at the same time

Notes
It can still wear real equipment in this slot. Both will render at the same time, see turtle shell example.