Nether/hopper item dupe

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

Steps to reproduce:

1. In the Nether, 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 using a Nether portal. (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).