They have the same access to the stuff we do if not more than we do. They have numbers and if the dupe is more productive and faster than any other method they know they will do it even if it takes time doing PvE/PvP tasks.
I am sure you are well aware of the "Trade Exploit" that was posted here in the Exploit section at the start of 4.3. That exploit was fixed before hand but only the dupe part.
Youtube Link:
RAoV Dupe Method "Trade Exploit" - YouTube
Here is an explanation I posted before of what was possible with Desync from Servers.
This used to work during 4.2.2 with turning Small Heavenly Shards into Heavenly Shards; as well as with crafting items such as True Gold. With Archaeology you could make items like (Word of Empress Zoe)
Word of Empress Zoe - Item - World of Warcraft again and again, without using any of the mats and they would combine into stacks of 20. That's 375(Gold)x20 per stack, which would only fill up one inventory slot. That's 7500(Gold) per slot in your in bags; so you can get the largest bags in the game which I think is 65-70 slots in total. So 65x7500=487,500 Gold every 15 minutes by only selling to vendors.(You also have to factor in the part where you would delete one off of each stack)