Valhelsia 3.

Perk for transfering items in inventory combined with enchantment which does the same allows duping blocks. (Vicio path perk "Spatial Manipulation" + Charm's "Acquisition" enchantment"

Update

Problem weren't in double enchantments, but cause of Scarf of luck item from "Artifacts" modification. There's no dupe when use this scarf with enchanted pickaxe, so I guess issue is in Astral Sorcery.

What version of the mod are you on? This might be a duplicate of #1616

I can't replicate this under the latest version of Valhesia 3. I'm pretty sure this is the same already fixed issue.