hopper duplicate bug

What I expected to happen was...:*

I wanted to build a monster trap, where 2 hoppers collect the items and do purely in a box.
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What actually happened was...:

I threw a item in the hopper for testing ( a stone ).
Then I looked in the chest then there were two stones inside.

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Steps to Reproduce:
1. Place a double chest.
2. Place 2 hoppers above them.
3. Threw a Item inside a hopper.
4. Look in the Chest.