item duplication

i was trying to create an item transport with a hopper, redstone comparator, and a dispenser

but what happened is my items has been duplicated from 64 to 101 [37 items were duplicated]

Steps to Reproduce
1. place alot of hoppers stacked on top of the dispenser
2. place a redstone comparator behind the dispenser and switch it on with a lever
3. shoot alot of item into the hopper slowly [around half a sec, good timing duplicates alot] [i use 64 diamonds]
4. collect all the items coming out from the dispenser
5. take a look on your inventory