The Duplex potion has the "you killed two monsters in one" effect. So items that are 100% loot will be doubled, for example unique items from Pits of Inferno bosses (piece of massacre's shell, imperor's trident, etc). Bosstiary's bonus increases the chance of dropping any equippable items, but does not double the items.
Remembering that the Duplex Potion does not work on bosses with a reward system, only those with loot for those who did the most damage, for example the bosses of PoI, Man In The Cave, Dharalion, Fernfang
An example of when I used Bonus and Duplex on Imperor. He gave 2 trident, the duplex effect. The bonus does not affect unique items, only equippable items (crown legs):