Yes, I know how it works. I played probably an aggregate of 20k hours into the Diablo franchise (I quit on Blizzard at this point though... such a shame what they've become). Vast majority of that time into d2.

You drop item you want to dupe on ground walk away a little bit, you pick up item from belt at same time that you pick up item from the ground. The pathing gives you time to move your mouse around. If you get the timings right the item dupes into the belt item. Original item is placed into your inventory and duped item stays in your "hand". Bam 2 items. You can't really patch this out. It's an engine issue. There's a delay in how it processes the action of picking up an item that uses the same buffer as "held" items. And since it's using the original shareware mpq, or whatever mpq you provide it... and blizzard never fixed this bug in any official MPQ... you're not going to see it patched unless you do something like run the fancy modern update versions. Blizzard's "fix" was to track item ids and if multiple were on the ground at the same time, delete one.

Worked fine for me... Took base sorc, duped staff with the second mana potion you get.