I'm not sure how exactly this works, but on Stoneblock2, I was able to dupe chickens using an animal net in conjunction with a Chicken Catcher (from Roost, a similar mod. TBH don't know why both are in the pack but whatever) in the offhand. I found it when I noticed that, whenever I use the animal met on something while I have something else in my offhand, both items act like I right clicked them, including interactions with the entity I'm picking up like feeding seeds to a chicken. This piqued my curiosity, so I started searching for more exploitable interactions, and surprise surprise, I found this. Any idea why this is happening? Shouldn't the animal net remove entities either at the exact same instant or before registering them in its NBT? Or, at the least, check that nothing else is also trying to remove the entity? And why is it possible to use the offhand at the same time at all?
So far I've only tested this with the Chicken Catcher, but I suspect it will also happen with items like Golden Lassos (Extra Utilities 2), Soul Vials (Ender IO), and so forth. It might even happen with other interactions that don't involve actually duping the entity, but changing it/transforming it/etc.