From Guild Wars Wiki
Duping is a process that by exploiting a flaw in the game, a cheating player (a "duper") creates many copies of an item. Duping or knowingly selling a "dupe" (which is slang for an item created with this exploit) is against the EULA and will result in an account mark, but much more likely just an outright and permanent ban.
Effects of duping
The most obviously affected items in Guild Wars are high-end miniatures and Armbraces of Truth. Due to the actions of players using duping exploits, many high end miniatures were permanently lost when their accounts were banned.