When hoppers are facing each other and in two different chunks. You load and unload(go through nether portal) several times, it will cause the hopper to speed up. The hopper can reach roughly 3 times as fast as the original speed(2.5 item/s)
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And this cause item storage system to leave items in all vertical chests too causing very inconsistent timing problems.
Also this breaks hopper timers.
Is this still an issue in the current Minecraft Snapshot 15w47c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
So that duplicates the item in the hopper?