Yes, I did check Tails' comment and I did see that the resolution says that this a duplicate but what bothers me is the fact that it says that the status is "resolved" when I can clearly reproduce this bug countless times.
People, I am very concerned that I posted this when I downloaded the 1.5pre-release, no one has responded, and the fact that this bug has NOT been fixed in the official release of 1.5. I am also concerned about the fact that someone has marked this bug as "resolved" when it is obvious that this bug has NOT been fixed.
OK, I tried this again. First i removed all the items from the hopper. Then I cleared my inventory with /clear. After that, I put 3 stacks of hoppers in my survival inventory. Note that this duplication did not happen when I put the stacks in my hot-bar. Then I open my inventory, ctrl-z, and opened the hopper GUI. There were 3 stacks of hoppers, the first already transferring. The 3 stacks of hoppers reappeared in my inventory.
Oh. Thanks! I didn't know that.