Bottle Duplication (commonly referred to as "bottle dupe" or "duping") is a technique that allows you to replace items in Link's inventory with a bottle. This may seem useless at first, but it is very helpful when collecting the Zora Eggs in a speedrun, since you then only have to collect one legitimate bottle. It is also a prerequisite to performing Bottle Adventure.
If performed correctly, either a 0 item count sound will play, Link will put on a mask, or the Lens of Truth will turn on, and then Link will empty out the bottle. After this animation has played, the item or mask will be replaced with an empty bottle.
† You can mash the start button here if you find it easier. Without holding R, start must be pressed 6f after A; with R held, pressing start 6f or 7f after A works.
†† See frame data described in method 1 above.
This video demostrates all 3 methods described above in order:
See the Equip Swap page for details.
"Item crap" refers to using bottle duplication to preserve item count after Song of Time. For instance, if the stick slot on the pause menu has a stick in it, the stick count is set to 0 when Song of Time is activated; however, if the stick slot does not have a stick in it (e.g. if a bottle has been duped in that slot), Song of Time does not set the stick count to 0. If stick is equip swapped onto a c-button prior to duping over the slot, then sticks can still be used after Song of Time. Even if stick is not equip swapped, getting another stick after Song of Time will put the stick back on its slot and the stick count will be whatever it was beforehand plus one. This constitutes "stick crap," and this video demonstrates it.
The ESS method is the most convenient to perform if bottles are required, as it can be performed anywhere. However, only a limited number of items may be duplicated over this way. The curiosity shop method is useful as not only can any item be duplicated over, but rupees are gained each time the trick is performed and the item isn't used up (up to 200 rupees if a valuable item is used). This makes it a great source of rupees, especially for 100% speedruns in which 5000 rupees are required for the bank heart piece.
Of the two Curiosity Shop methods, the first method is a general method that will always work. The second method can only be used if the C-button item in question can be used inside the shop. This means it is mostly limited to duplicating over masks and other non-offensive items like the Ocarina or the Pictobox whereas the first method may duplicate over any item which can be equipped to the C-buttons.
Using collection delay makes the curiosity shop version of the trick extremely easy, but requires at least two bottles.
Using the same method as the Curiosity Shop versions of the trick, it is also possible to duplicate bottles in the Magic Hags' Potion Shop by depositing a Magic Mushroom. The trick works in the exact same way, but the downside is that it requires the Mask of Scents.