Bottle Duplication (commonly referred to as "bottle dupe" or "duping") is a technique that allows you to replace any item 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.
† You can mash the start button here if you find it easier
†† Again, mashing the C button is also an option
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.
Using collection delay makes this trick extremely easy, but requires at least two bottles.
Though all of the examples above show the Curiosity Shop, 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.