I was looking for a way to collect items with the hopper from an auto farm and came across a bug/glitch/item dupe. I'm not sure if it has to do with the amount of hoppers that are being used or not that are connected together to a single chest. I have been able to double the amount of items. I'll have a YouTube video up shortly to show this.

I was able to duplicate this in SSP and SMP whether it is in Creative or Survival.