Seems, this glitch fixed in Bukkit (as of 1.4.5-R0.2), but still works in Vanilla 1.4.5
As 1.3 did not fix this glitch, I make (very) simple plugin that prevents it.
How it works: When piston push sand or gravel block, it is marking. And when marked piston extends/retracts again, plugin checks delay between previous push. Sand/Gravel duplicators do extending/retracting very quickly, and plugin block that.
Because plugin checks only sand and gravel blocks for instant extend-retract, it should not affect 99% of piston mechanisms
Also prevent DragonEgg duplication (thx Ribesg)
