Hopper: item duplication

Items dropped into a hopper "production line" ended up multiplying.

What I expected to happen was...:
Items dropped in a hopper would travel down a line of hoppers, into a chest.

What actually happened was...:
The above did occur, but more items were in the output chest than were dropped in.

Steps to Reproduce:
1. First, set up a hopper to chest chain like this:
XX
X
XC
Where X = Hopper and C = chest. (all joining to adjacent blocks)

2. Drop a load of items into the top-right hopper, "spamming" with items.
3. Open the chest. Quite often, you will find more items in the chest than you dropped in.