Well I'm like 90% sure our silver didn't come from an item dupe, considering the guy we attacked had 500,000+ crates of nothing but peasant gear (and like 2.7 billion dresses that at one point cost 0 silver each, and post-patch were worth like 30 silver each). Just a really good case of dumb luck, and all we've done with it since then is give it away and supply our armies with themed gear.
I know that Frisia encountered a true item dupe back when we were all Fimbulvetr, and I've heard rumors that Occitan and Semenstorm have also had cases of item duplication, but not on the same level as the Frisian example. Murdertron and I had an XP battle awhile back and had an item dupe that supplied a shit ton of varangopoulos armor, which like I said before sold for around 5m in my case and supplied him with a shit ton of armor for his future XP battles.