Item drops (such as rupees, hearts, arrows, etc) can be duped by touching a drop while either holding the shield out, holding the sword out, or both. The theory behind this trick is that the drop is touching both Link's body and his sword/shield at the same time, and thus both collisions are recorded at the same time, duplicating the item. Drop duping is generally subject to luck, not only from the drop itself but also because of the global timer. Because of this, a drop can only be duped about 50% of the time. Likewise, drops can only be duplicated once - holding the sword and shield out will not result in grabbing 3 items, only 2.
Duping from bushes is very simple. Walk up as close as possible to a bush from any side, slash it, and walk towards the bush. The directional input should begin during the slash.
To dupe a drop from an enemy, kill it and hold the shield and/or sword out. Walk over to where the enemy's death animation is occuring and center yourself. If you're lucky, you should get two item drops.
Note that for this method it does not matter which way Link is facing. So long as you position yourself on top of the rupee, a drop dupe should always have a chance of occurring.
The above methods all rely on the item drop finishing its bouncing animation while Link is standing on top of it. Dry drop duping is a method of duping that does not rely on this. To perform a dry dupe, walk into the item and press the shield button on the same frame Link touches the item. This method is likely the hardest as its the only method that requires some skill in order to pull off.
There is a way of setting this method up as well (demonstrated with a rupee). Line yourself up in one of these four positions and press the direction towards the rupee as well as the shield button on the same frame. Note that for the positions to the left and right sides of the rupee, Link's vertical position does not matter. For positions above and below the rupee, his horizonal position does not matter.
|Dry dupe setups|
Dry dupes can be done from the sides while Link is facing up or down as well - diagonal inputs will be needed in order for them to have a chance of working.