If you put armor pieces into dispensers (or right-click with them in your hand), then they will put the armor onto the player incorrectly.

It has nothing to do with weather canPickUpLoot is set to 1 or 0

It is not a duplication glitch of any kind, simply automatic armor placement is botched.

What should of happened
The helmet goes into the helmet slot, the chestplate goes into the chestplate slot, the leggings go in the leggings slot, and the boots go into the boots slot.

What actually happens
The helmet goes into the chestplate slot, the chestplate goes into the hotbar, the leggings go into the boots slot, and the boots go into the hotbar.

How to reproduce
Place 4 dispensers, and put 1 piece of armor in each(helmet in one, clestplate in another, etc), activate the dispensers (by putting a pressure plate in front of the opening) OR get the armor pieces into your hotbar. Right click each piece after selecting it (in survival)