Hopper Clocks Duplicating Items

It is possible to duplicate any obtainable item, while preserving any metadata like enchantments.

Steps to reproduce:

1. Build two hoppers facing into each other. (Use more hopper pairs to increase the probability.)
2. Put one item that you want to dupe in one of the two hoppers.
3. Unload the chunk(s) that the hoppers are in, for example by leaving the area or using a Nether portal, then return to the location of the hoppers. (Repeat for more dupes.)
4. Some items will have duplicated. (For some reason, it doesn't always work.)

This bug seems to be related to lag, so on slower servers the dupe will be more productive.

The bug is shown in this video (which has over 400 000 views, so it might make sense to make this issue public).

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(This ticket was originally reported by Jason Purslow. Original description below.)

After creating a clock with 2 hoppers linked together with one item put inside one of them, teleporting to and from (or leaving and returning to the area) will sometimes result in the item placed inside one of the hoppers being duplicated.

I have replicated the bug in this video: http://www.youtube.com/watch?v=aOVGH0_nRUw