Duplication of item in hopper clock

When I create a one-item hopper clock, occasionally the single item in the clock duplicates itself, breaking the clock and producing two copies of the item.

What I expected to happen was...:
There should be no duplication.

What actually happened was...:
There was duplication.

Steps to Reproduce:
1. Set up a hopper clock. I used bonemeal as the item in the clock, I don't know if that's significant.
(To check the bug: )
2. Set up a comparator output into a command block continually producing an output.
3. Run far away (until the clock is unloaded) and then run back again. Repeat continuously.
4. Eventually, the clock will no longer work, even if you're in the chunk containing the clock. Check the hoppers, and there'll probably be two items in one of the hoppers.

This will only check if the item is duplicated into one of the hoppers (the one that the comparator is running into), but not the other one. However, messing around suggests that it can be duplicated into either. Running away and back again also tends to send the duplicated item into the other hopper, making the machine run again temporarily.

If it matters, the hopper alignment that broke for me ran parallel to the x-direction (that is, both hoppers had the same z-coordinate).