This trick currently has no use in a speedrun
It is possible to duplicate items picked up off the ground using certain cutscenes. If an item is picked up on the same frame as a switch is activated, you'll pick up the item as the cutscene is playing. You'll get the item and a duplicate will be left on the ground under you.
This only works with items that give descriptions after being picked up. This should be technically possible with any "success" cutscene such as blowing up a wall, hitting a switch, opening a door etc. but would too hard to pull off in a speedrun.
The easiest way to Item Dupe would be to unmerge onto a switch that has an item on top of it. This can be seen in the first room of House of Gales. There are pots covering a switch that activates the fans in the room. If you break the pots and a Monster Gut spawns on top of the switch, merge along the wall and unmerge onto the switch.