Terraria Ore Excavator duplication glitch
This works by holding the block you want to dup, let's say it's gold ore, with the mouse an than using the ore excavator "swap" function to swap the blocks like platforms (or any other block works too) with the gold ore you are holding, this will only consume 19 gold ore from the stack you're holding and still place the original stack amount. Meaning if you hold 200 gold ore and duplicate it than you'd have end up with 381 gold ore (200 - 19 + 200).
You can start to duplicating as soon as you have at least 20 of any block.
The block you want to duplicate needs to be one that can be "block swapped" in order for this to work.
The reason for using platforms is, since they have the ore excavator "place" function you can replace them fast and easily making them the best option for duping fast.