File Duping is a way of copying over file information from one save file to another during BiT. When playing the game, all the information of your file is loaded on the "Active" File in memory. By saving the game, the "Active" File information is saved onto the "Targetted Savefile Slot", which can be Slot 1, 2 or 3. The targetted file slot is normally the slot number of the last file that was manually selected in the Title Screen menu, which means in normal gameplay it is the slot of the file you are playing on. However, in Back in Time it is possible to desync the "Active File" and the "Targetted Slot". This is File Duping.
By saving during BiT without having a file selected in the file menu, after opening the splash screen, the file with the highest possible number will be copied over the file that was last manually selected during the BiT. This is because opening the Splash Screen updates the "Active" File when it comes to saving, but not the "Targetted Slot".
If no file was selected previously during the BiT, the game defaults to copying the file with the highest number over the file that activated BiT. In other words, this glitch allows you to copy file 3 over to another file slot of your choice. If file 3 doesn't exist (ie it is a new adventure file), it will copy file 2, and if that doesn't exist, it will copy file 1. If no file exists, it will generate a BiT save file.
File Duping has interesting properties with new games files, both "New Adventure" and "Hero Mode".
If you overwrite an empty file with a file that has made progress, the file will still say "New Adventure" or "Hero Mode" even though the file exists. With a New Adventure file, opening it afterwards will reveal that it is a perfect copy of the file with the highest slot number, BiTSaved where the file duped was done. With a Hero Mode file, things are slightly different: you have one chance to load it into the intro cutscenes, and Link will appear at his bed afterwards, but with all the items/quests from the file that was copied over the hero mode file, with the exception that the number of hearts will be set to six. If you go back to file select after selecting the file duped Hero Mode file, if you copy, delete or go back to the splash screen, you will lose that chance, and instead the Hero Mode file will be BiTSaved where the file dupe was done, just like what happens with a New Adventure file.
Duping a New Game file over another file is also interesting. For this, you will need an empty Hero Mode file on slot 3 (you can not dupe an New Adventure file over another file, only Hero Mode can because selecting it to load its information is possible). Activate BiT. Select File 1 or 2 then go back to the Splash Screen. Perform a BiTSave while opening the Splash Screen to File Dupe file 3 over file 1 or 2, then open that file. Now you can do as many things as you want, like obtaining items. If you save and quit you will get to keep all of your progress, but that file will will say Hero Mode, and whenever you load it, the intro cinematic videos play and then Link will be at his bed in Skyloft. Once the intro cutscene has been watched, saving gets rid of the new file property.
Both of these tricks are useful to give the save prompt property to any file of your choice. Being saved at a save prompt (which both New Adventure and Hero Mode are) allows Reverse BiTWarps to act differently. The first method is useful for exemple to setup Faron BiT with a Sky RBW. The second method is generally less useful due to it erasing all of your progress first.
Using the file duplication glitch, it is possible to beat the game in under seven minutes. You need a Hero Mode file saved before (or after) the Demise fight at the end of the game, and this file should be in slot 3. You can start with a blank Hero Mode file in either slot 1 or 2. The route is as follows:
Note: This is not a legal speedrun, as it requires a file to be already made. This glitch is simply an elaborate way to copy a file.