Item Duplication in Hopper chains

Once, I put a hoppers in a chain. When I threw a stack of items (Sandstone) quickly one by one, into the hopper at the end, it should've just passed through the hoppers.

However, it ended up duplicating the sandstone.

How to Reproduce:
1. Put a chain of hoppers connected to each other.
2. Throw a stack of sandstone in the hopper at the end.