Did not find anything like it so lemme try..
Needed:
1. Strider
2. Lots of stuff u wanna dupe
3. Friend
Rules are simple, u can dupe only items that stack in one place in ur inventory. For example.:Adena, AA, Mantra/Seal Stones, Mana Potions, HP Potions, CP Potions..u get the idea
Process:
1. Summon ur strider in some less crowded town, FE.: (Heine).
2. Now u need to make this thing really fast, when ur friend asks u for a trade u accept the trade and press TAB so fast that the trade and ur inventory opens at once.
3. U must have Trade, Inventory, and pets Inventory slots open.
4. Lets say we dupe Adenas, put 25 KK Adena in the trade, ur friend puts 25 KK Adena aswell.
5. Put 1 Adena in ur pets Inventory.
6. Put 1 Adena in the Trade again.
7. Now at this momment u should not have any more adena in ur inventory, the trick is ur already bugged at the trade..it shows 0 Adena in ur inventory but u still but 25 KK in ur pets inventory, do it all like its said and there should be 25 KK and 1 Adena in Trade and Pets Inventory.
8. Cancel the trade.
9. Now u see u have 25 KK and 1 Adena.
10. Unsummon ur pet, still 25 KK 1 Adena.
11. Delete 1 Adena and u get double u had.. repeat as many times as u want, just remember u and ur friend must have the same ammount of Adena.
Any questions ?