There have been few confirmed ways to duplicate items in Tibia, despite how often players claim they have done it.

Black Knight Door Trick

There was a claimed way to duplicate items using the Black Knight door trick. (Please edit and add information on this trick if you have it available, I have searched around and only found mentions of this trick, with no formal explanation.)

House Duping

There was another way that item duplicating was discovered. This was on the server Nova, by Messiah (name changed to Ceodriz)?, during one of the first great Nova wars. Messiah (Ceodriz) discovered this trick in his house, during a server save (presumably on accident). The trick worked like this: one would wait until just moments before server save, and then quickly throw an item on the ground. On Nova, server save at the time was at 3:00:00 AM EST (GMT +5), so you would have to wait until approximately 2:59:30 AM EST. If the dupe was successful, you'd find the item both in your inventory and on the ground of your house after server save.

This is most likely because character inventories were saved just before the server save "kick-off" would occur, and house decorations would be saved shortly thereafter. Supporting evidence to this is that after the "fix" was applied, characters would stay logged in during the server save, even though the players themselves were kicked off. It's assumed that the character inventories would not be saved until the player was no longer actively in the game, to prevent these types of duplications from occurring.

It was through this method that Messiah was able to have so many Amulets of Life and Backpacks of Mana Potions. This enabled him to have the highest magic level in Tibia at the time (76). It also contributed to earning him the nickname "The Human Mana Cask".