Duplication of items dropped on the hopper.

When throwing an item into a hopper f.e. Diamonds, they get duplicated. Tried both on creative and survival. Does not work with blocks
Steps to reproduce:
1. Place a hopper on the ground.
2. Throw a stack of anything on the corner of the hopper.
3. The items will still be in the corner but hopper will start filling itself with the item you chose, in this example stack of gold.