Duplicate glitch - Anvil (don't close, please!) [UNSOLVED!]

A video explaining the glitch:
http://www.youtube.com/watch?v=2NevsGk77kE&hd=1

I play on a sever with Shop Plugin, where players can buy and sell using sign. Buying more than 1 sword, by example, it becomes stackable. The server had to disable the anvil because of that, and that's not cool.
I don't use any other mod, my folder is clean. I'm only using a texture pack, which does not affect the process of the bug.