Bottle Duplication allows you to overwrite one of your items with a Bottle, effectively giving you a new bottle to use. The item you overwrite may be permanently deleted, however. Duplicate Bottles are not recognized when obtaining potions, Poes, or Milk, and a Fairy in a duped Bottle will not revive Link automatically. This does not apply to bottles duped over the child and adult trade quest items, however.
To be able to perform this, catch something in a bottle, then backflip (or any kind of jump) and press the bottle button and then a item like (nuts, sticks, lens of truth, masks, etc). The item will get duplicated into a new bottle.
You can perform Bottle Dupe by holding ESS position and doing the same as backflip method, press bottle button after catching something and the item you wanna dupe over.
Normally shield swipe duping does not work, due to the fact that you are not supposed to be able to target shield and have an item in hand. However, by exiting the ocarina and quickly shielding and targeting, it is possible to meet these conditions. Therefore, if you exit ocarina, shield, hit the button for the item you want to dupe over, then target in very quick succession, you store the item you pressed while shielding as your last input. By holding the empty bottle button that's in your hand and releasing shield to catch an item, you then dupe a bottle over the item. If you do not hit a button for an item to dupe over, the game automatically tries to dupe over ocarina. However, since ocarina does not have a bottle offset slot, you end up duping over deku sticks.