Issue:
Trades where a villager accepts an item which has variable metadata or NBT data can enable a dupe exploit when clicking the "trade" button while having more items in the inventory. See the following images for an example of what's happening:

This dupe issue also works with stack-able items that differ in NBT data, such as rustic elixirs or vanilla potions w/ quark.

Walkthrough:
This villager accepts Immersive Engineering "tough fabric" and gives coins. Tough fabric has an item ID of "immersiveengineering:material" and a metadata of "5". In this example, a player places the item "Iron Mechanical Component" into the trade slot - a tough to acquire item which has the same item ID as the tough fabric but a metadata of "8". The player also has 63 "Hemp Fibers" in their inventory, an easy to acquire item with the same item ID as the previous items but a metadata of "4".

The player then clicks the trade GUI button for the tough fabric trade. This causes every stack of hemp fiber in the inventory to transform into iron mechanical components. The player does not actually need tough fiber and never needs to actually even make a trade.

Versions Used:
Minecraft version 1.12.2
FutureMC version 0.2.8
Forge version 14.23.5.2860

Good timing