When you put 2 hoppers one next to the other and the tubes are united, you can duplicate whatever you put there!
STEPS:
1)Put 2 hoppers with the little tubes united
2)Put something into A hopper
3)Open a hopper's GUI
4)Spam the left mouse button on the first free space
5)Take whatever you put there out
6)Throw that on/next to(try both)a hopper
7)Open again a hopper's GUI
8)If you want to take it out of the hopper, then hold shift(or your sneak button)and spam again the left mouse button the first space of a hopper
9)DONE!