If you click inside a pyramid with a flint and steel in your main hand and have any item in your off hand, it dupes the off hand item but only client side.
Can go beyond 64 even: https://cdn.discordapp.com/attachments/692817077626470481/842221754306920478/2021-05-12_21.08.15.png
Minecraft version: 1.16.5
Atum version: Atum-1.16.5-2.2.0