Duplication Bug

When I shot an arrow on myself and died from it, I respawned with my chestplate still equipped and when I went to retrieve my stuff, A SECOND ONE was in the pile of my items with the same durability. This has happened before (And with a different type of item), A bit before the latest version came out But Both times I didn't know how to do this on purpose. Can you guys give me a bit of info on how to fix this or better, how it happened?