So I filled inventory like this, ghostly pickaxe, one set of blocks to duplicate (diamond blocks) and other items (bricks)

get extra diamond blocks and place it. the inventory should remain full. then mine at least two diamond blocks with ghostly pickaxe.

when you place a couple of diamond blocks, you can see it duplicates itself. the diamond blocks just keep filling themselves up so they never run out.
this duplication glitch works with other blocks that can be stacked up like iron blocks, and planks

also my english is pretty bad cuz it's not my native language

No fixed in Alex's Mobs

I've also been able to replicate this bug with any block, shulkers too (so you can dupe anything). It works on stackable blocks normally and if you use the "Digging claws" from the Artifacts mod that give you faster mining speed (which I'm assuming is what makes it work for non-stackable blocks) it works on any block. This was Alex mobs version 1.21.1, on forge 1.19.2-43.2.6 (artifatcs was version 5.0.2)

Fixed in Artifacts version 5.0.4. To prevent similar issues with other mods, you should probably give your HarvestCheck event handler a lower priority.