When using GemShears from GwyCraft, leafs duplicate when mined. i.e You mine one leaf block with the shears and it drops 2. The shears do not have Fortune. Only Silk Touch. It isn't game changing but I'm fairly sure it isnt meant to happen
Sorry if this is an actual game mechanic, I just thought it was odd.
It would become a potential problem if you could place the leaves and dupe them again - can you? That would only really be a problem in any mods that use leaves, like Ex Nihilo where you make dirt from them though.