Hopper facing downwards to another hopper, gives 2 items at a time.

One item at a time should be transfered

Transfered two items at a time

Steps to Reproduce:
1. Place a hopper on the ground.
2. Place another hopper above the other by shift-right clicking.
3. Put a stack of items inside the top hopper.
4. Watch them as they flow two at a time.