On the subject of duping, you can't say its not serious as 2 maps ago the server was filled with duped blocks, it made mining almost pointless. When the game was patched we generated a new map on the server to start clean, playing as intended. This is the way most of us like to play the game, and if the admins also want to keep it this way then that means they have to do extra work to stop people who dupe and clean up after them, so anything that stops duping before people get the chance to do it, or punish those who already did, is alright in my opinion.
That said I do think a permaban is a little harsh, by the sounds of it its not like you tried to conceal the fact that you found a way to dupe (which would have been worthy of a permaban), but I'm not a mod so my opinion on this doesn't realy change anything.