When you put multiple diamond equipment items to the slot in the smithing table to upgrade it to netherite you only get on of the netherite item, but consume all diamond items in the first slot.

How to reproduce:

 1. Summon an armor stand that has 64 diamond boots. Use this command: /summon minecraft:armor_stand ~ ~ ~ {HandItems:[

]}

2. Take a netherite ingot.

3. Put the 64 diamond boots and the netherite ingot in the smithing tabe.

4. Take the netherite boots out.

You will only get one netherite boots but consume all diamond boots.

I don’t know another way to get stacked diamond armor and I also don’t know if it is intended that you can get more than one of unstackable item on a slot using this trick.